Added description for each flag.

help.md

@@ -1,2 +1,96 @@
 ### RTLAMR Help
 Detailed usage information for the various flags of RTLAMR.
+
+  - `logfile` writes log statements to the given file. Defaults to `/dev/stdout`.
+  - `samplefile` writes raw signal to the given file. Samples are interleaved 8-bit inphase and quadrature pairs. Fields Offset and Length are omitted in the plain log format if this option isn't used. Defaults to `/dev/null`.
+  - `cpuprofile` writes pprof profiling information to the given filename. Useful for determining bottlenecks and performance of the program. Defaults to blank and writes no profiling information.
+  - `duration` sets the amount of time to listen for before exiting. Defaults to 0 for infinite, [GoDoc: time.Duration](http://godoc.org/time#Duration)
+  - `fastmag` uses a faster magnitude calculation algorithm, sacrifices accuracy for speed. Defaults to false.
+  - `filterid` display and dump raw samples only for messages with a matching meter id. Defaults to 0 for no filtering.
+  - `filtertype` display and dump raw samples only for messages with a matching type. Defaults to 0 for no filtering.
+  - `format` format to write log messages in. Defaults to plain. Options: plain, csv, json, xml or gob
+  - `gobunsafe` allows gob output to stdout. Gob output is not stdout safe and will bork a terminal so user must specify `-gobunsafe` or specify a non-stdout file via `-logfile`. Defaults to false and warns user.
+  - `msgtype` specifies the message type to receive: scm or idm. Defaults to scm.
+  - `quiet` suppresses printing state information at startup. Defaults to false.
+  - `single` will listen until exactly one message is received that matches all of the given filters if any. Defaults to false.
+  - `symbollength` sets the symbol length in samples. Defaults to 73.
+
+    Sample rate is determined by this value as follows:
+
+    ```
+DataRate = 32768
+SampleRate = SymbolLength * DataRate
+    ```
+
+    Sample rates are limited by the dongle such that:
+
+    ```
+225 kHz < Sample Rate < 300 kHz
+900 kHz < Sample Rate < 3.2 MHz
+    ```
+
+    The symbol length also determines the size of sample blocks read and processed on each pass.
+
+    ```
+PreambleSymbols = 21 (for SCM) and 32 (for IDM)
+BlockSize = 1 << uint(math.Ceil(math.Log2(float64(PreambleSymbols * SymbolLength))))
+    ```
+
+    Valid symbol lengths are given below, block size calculated for SCM:
+
+      - Block Size: 512
+
+      Symbol Length | Sample Rate
+      ------------- | -----------
+      7             | 229.376 kHz
+      8             | 262.144 kHz
+      9             | 294.912 kHz
+
+      - Block Size: 2048
+
+      Symbol Length | Sample Rate  | Symbol Length | Sample Rate 
+      ------------- | -----------  | ------------- | ----------- 
+      28            | 917.504 kHz  | 39            | 1.277952 MHz
+      29            | 950.272 kHz  | 40            | 1.310720 MHz
+      30            | 983.040 kHz  | 41            | 1.343488 MHz
+      31            | 1.015808 MHz | 42            | 1.376256 MHz
+      32            | 1.048576 MHz | 43            | 1.409024 MHz
+      33            | 1.081344 MHz | 44            | 1.441792 MHz
+      34            | 1.114112 MHz | 45            | 1.474560 MHz
+      35            | 1.146880 MHz | 46            | 1.507328 MHz
+      36            | 1.179648 MHz | 47            | 1.540096 MHz
+      37            | 1.212416 MHz | 48            | 1.572864 MHz
+      38            | 1.245184 MHz
+
+      - Block Size: 4096
+
+      Symbol Length | Sample Rate  | Symbol Length | Sample Rate 
+      ------------- | -----------  | ------------- | ----------- 
+      49            | 1.605632 MHz | 74            | 2.424832 MHz
+      50            | 1.638400 MHz | 75            | 2.457600 MHz
+      51            | 1.671168 MHz | 76            | 2.490368 MHz
+      52            | 1.703936 MHz | 77            | 2.523136 MHz
+      53            | 1.736704 MHz | 78            | 2.555904 MHz
+      54            | 1.769472 MHz | 79            | 2.588672 MHz
+      55            | 1.802240 MHz | 80            | 2.621440 MHz
+      56            | 1.835008 MHz | 81            | 2.654208 MHz
+      57            | 1.867776 MHz | 82            | 2.686976 MHz
+      58            | 1.900544 MHz | 83            | 2.719744 MHz
+      59            | 1.933312 MHz | 84            | 2.752512 MHz
+      60            | 1.966080 MHz | 85            | 2.785280 MHz
+      61            | 1.998848 MHz | 86            | 2.818048 MHz
+      62            | 2.031616 MHz | 87            | 2.850816 MHz
+      63            | 2.064384 MHz | 88            | 2.883584 MHz
+      64            | 2.097152 MHz | 89            | 2.916352 MHz
+      65            | 2.129920 MHz | 90            | 2.949120 MHz
+      66            | 2.162688 MHz | 91            | 2.981888 MHz
+      67            | 2.195456 MHz | 92            | 3.014656 MHz
+      68            | 2.228224 MHz | 93            | 3.047424 MHz
+      69            | 2.260992 MHz | 94            | 3.080192 MHz
+      70            | 2.293760 MHz | 95            | 3.112960 MHz
+      71            | 2.326528 MHz | 96            | 3.145728 MHz
+      72            | 2.359296 MHz | 97            | 3.178496 MHz
+      73            | 2.392064 MHz
+  - `centerfreq` sets the center frequency to receive on. Defaults to 920299072.
+  - `samplerate` sets the sample rate. This will override the sample rate calculated by `-symbollength`.
+  - If any of the gain-related flags are specified rtlamr won't set any gain options of it's own. By default rtlamr enables `-tunergainmode`. Flags which disable this behavior: `-gainbyindex`, `-tunergainmode`, `-tunergain` and `-agcmode`.