test/codegen: add README file for the codegen test harness
test/codegen: add README file for the codegen test harness

This change adds a README file inside the test/codegen directory,
explaining how to run the codegen tests and the syntax of the regexps
comments used to match assembly instructions.

Change-Id: Ica4eb3ffa9c6975371538cc8ae0ac3c1a3a03baf
Reviewed-on: https://go-review.googlesource.com/99156
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

CAFxX/go,akutz/go,josharian/go,golang/go,josharian/go,cloudflare/go,CAFxX/go,codestation/go,Cofyc/go,Cofyc/go,codestation/go,nawawi/go,codestation/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,Cofyc/go,codestation/go,codestation/go,Cofyc/go,golang/go,cloudflare/go,CAFxX/go,josharian/go,akutz/go,akutz/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,akutz/go,codestation/go,Cofyc/go,nawawi/go,josharian/go,Cofyc/go,Cofyc/go,akutz/go,CAFxX/go,Cofyc/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,golang/go,akutz/go,akutz/go,CAFxX/go,josharian/go,cloudflare/go,josharian/go,nawawi/go,codestation/go,akutz/go,golang/go,codestation/go