[direnv] Add basic .envrc.example (#4747)

Add a basic .envrc.example that will build the go sdk using the Makefile and
setup the environment to use it so developers who use direnv
can easily "automatically" use the managed go sdk. Update CONTRIBUTING.md to
add a section describing the intended usage of direnv .envrc.example.

Signed-off-by: Zack Train <ztrain@uber.com>

.envrc.example

@@ -0,0 +1,8 @@
+toplevel="$(git rev-parse --show-toplevel)"
+
+# build and use the managed go sdk
+unset GOROOT
+make -C "$toplevel" go-check
+PATH="$(make --no-print-directory -C "$toplevel" go-bin-path)"
+
+# add custom direnv initialization below here

.gitignore

@@ -1,6 +1,7 @@
 .build
 .cache
 .data
+.envrc
 .glide
 .tmp
 .DS_Store

CONTRIBUTING.md

@@ -233,6 +233,16 @@ implementation can easily serve the needs for an entire suite of tests and
 the behavior is in a centralized location when it needs to be updated. Fakes
 are also less inclined to be impacted by changes to usage patterns.
 
+## Example [direnv][direnv_link] .envrc
+
+We have committed a basic `.envrc.example`. If you use [direnv][direnv_link],
+copy it into `.envrc`, edit as desired, and enable it with `direnv allow`. The
+`.envrc` is `.gitignored`. Be aware that [source_env][source_env] is insecure
+so keep your customizations in `.envrc`.
+
+[direnv_link]: https://direnv.net/
+[source_env]: https://direnv.net/man/direnv-stdlib.1.html#codesourceenv-ltfileordirpathgtcode
+
 ## Git hooks
 
 We have checked in a pre-commit hook which enforces `go fmt` styling. Please install it