Update doc/compile/README.md
Added String#sub/sub! and String#gsub/gsub!
Merge pull request #838 from brainopia/patch-1

Fix typos in compilation guide
Merge pull request #839 from masuidrive/string_sub_gsub

Added String#sub/sub! and String#gsub/gsub!
Merge branch 'master' into iij

Conflicts:
	mrblib/string.rb
	test/t/string.rb