Document build flags that reduce binary size

[bmoffatt]

Using the -ldflags=-w flags to remove the debug symbols cuts the size of the binary in half too, seems strange that AWS isn't recommending this as well.

With any non-default build flags, there may be some tradeoff to understand and incorporate into any documentation. Dropping the debug symbols does seem risk free though.

Originally posted by @bmoffatt in #493 (comment)

There's also -tags lambda.norpc that could be documented.