Update emacs.md - add .dir-locals.el example
Update emacs.md - add .dir-locals.el example

Change-Id: I96c6c3498c47902b694ef0b141175826b7ebe689
GitHub-Last-Rev: bf03d6e5a33cc1984a76ef9b45d642947a07bb85
GitHub-Pull-Request: golang/tools#275
Reviewed-on: https://go-review.googlesource.com/c/tools/+/294110
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools