github: add a Pull Request template

Change-Id: I02938b2435e3a98efea7ee5545a6f8f5f6f794b4
Reviewed-on: https://go-review.googlesource.com/93915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/PULL_REQUEST_TEMPLATE.md

+This PR will be imported into Gerrit with the title and first
+comment (this text) used to generate the subject and body of
+the Gerrit change.
+**Please ensure you adhere to and check off each item in this list.**
+More info can be found at https://github.com/golang/go/wiki/CommitMessage
+
++ [ ] The PR title is formatted as follows: `net/http: frob the quux before blarfing`
+  + The package name goes before the colon
+  + The part after the colon uses the verb tense + phrase that completes the blank in,
+    "This change modifies Go to ___________"
+  + Lowercase verb after the colon
+  + No trailing period
+  + Keep the title as short as possible. ideally under 76 characters or shorter
++ [ ] No Markdown
++ [ ] The first PR comment (this one) is wrapped at 76 characters, unless it's
+      really needed (ASCII art, table, or long link)
++ [ ] If there is a corresponding issue, add either `Fixes #1234` or `Updates #1234`
+      (the latter if this is not a complete fix) to this comment
++ [ ] If referring to a repo other than `golang/go` you can use the
+      `owner/repo#issue_number` syntax: `Fixes golang/tools#1234`
++ [ ] Delete these instructions once you have read and applied them