cmd/compile: edit README
cmd/compile: edit README

Make some small edits for clarification of important concepts.

Change-Id: I1f78f2a59489ac71e3948dc924641540508613ce
GitHub-Last-Rev: 1819140ee607edf8be3ed35f846848178065391a
GitHub-Pull-Request: golang/go#50980
Reviewed-on: https://go-review.googlesource.com/c/go/+/382458
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go