Update README.md

Added a more complete description.

README.md

@@ -1,2 +1,42 @@
 # se_ms_api
  SolarEdge Monitoring Server API
+
+This library provides access to the SolarEdge Monitoring API.  The API is defined [here](https://www.solaredge.com/sites/default/files/se_monitoring_api.pdf),
+released January 2022.  I am in no way associated with SolarEdge.  And it follows that this software is not associated with SolarEdge.
+
+You can use this library to get data collected by your SolarEdge inverter.  I use it to get the amount of self consumption for a billing period.
+I then use this data to track how close I am to breaking even on my solar investment.
+
+I've implemented the API as defined, except for the bulk requests. Since I only have one inverter, I cannot test bulk.
+
+I did this as an exercise to learn Rust.  There are other similar libraries out there that are more sophisticated. 
+
+Due to the restrictions that SolarEdge imposes on this API, this library does not try to be performant. For example, it makes blocking HTTP requests.
+
+The basic use case is:
+
+1. Create a SolarEdge struct that contains the site id and api key that will be used for the requests. (The se_monitoring_api.pdf linked above has instructions for getting your site id and api key.
+2. Create a request for the information that you want.
+3. Send the request using the SolarEdge struct.
+4. Read the response to get the information.
+
+```rust
+extern crate se_ms_api;
+use se_ms_api::{SendReq, SiteDetailsReq, SolaredgeCredentials};
+
+let site_id = "my_site_id";
+let api_key = "my_api_key";
+
+let cred = SolaredgeCredentials::new(&site_id, &api_key); // (1)
+let req = SiteDetailsReq::new();                          // (2)
+let resp = req.send(&cred);                               // (3)
+
+match resp {                                              // (4)
+   Ok(r) => {
+       println!("My site's status is {}.", r.details.status);
+   }
+   Err(e) => {
+       panic!("Unexpected SiteDetails response: {:?}", e);
+   }
+}
+```