doc: add issue and pull request templates

Fixes #14365

Change-Id: I082329fe7a1e06c774a32e0e24e5c8736bb5a037
Reviewed-on: https://go-review.googlesource.com/19877Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/ISSUE_TEMPLATE

+Please answer these questions before submitting your issue. Thanks!
+
+1. What version of Go are you using (`go version`)?
+
+
+2. What operating system and processor architecture are you using (`go env`)?
+
+
+3. What did you do?
+(Use play.golang.org to provide a runnable example, if possible.)
+
+
+4. What did you expect to see?
+
+
+5. What did you see instead?
+
+

.github/PULL_REQUEST_TEMPLATE

+Please do not send pull requests to the golang/* repositories.
+
+We do, however, take contributions gladly.
+
+See https://golang.org/doc/contribute.html
+
+Thanks!