github: update Pull Request template

+ Move from Markdown checklist to text. The first PR comment is
  presented as text when creating it.
+ Add the note about Signed-Off-By: not being required.

Change-Id: I0650891dcf11ed7dd367007148730ba2917784fe
Reviewed-on: https://go-review.googlesource.com/95696Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/PULL_REQUEST_TEMPLATE.md → .github/PULL_REQUEST_TEMPLATE

 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.**
+
+**Please ensure you adhere to every 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 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
++ 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`
++ 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
++ 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
++ We do not use Signed-off-by lines in Go. Please don't add them.
+  Our Gerrit server & GitHub bots enforce CLA compliance instead.
++ Delete these instructions once you have read and applied them