doc: improve issue template and contribution guidelines

Encourage people towards the various help forums as a first port of
call. Better sign-posting will reduce the incidence or questions being
asked in the issue tracker that should otherwise be handled elsewhere,
thereby keeping the issue tracker email traffic more focussed.

Change-Id: I13b2e498d88be010fca421067ae6fb579a46d6b7
Reviewed-on: https://go-review.googlesource.com/34250
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/ISSUE_TEMPLATE

@@ -7,6 +7,7 @@ Please answer these questions before submitting your issue. Thanks!
 
 
 ### What did you do?
+
 If possible, provide a recipe for reproducing the error.
 A complete runnable program is good.
 A link on play.golang.org is best.

CONTRIBUTING.md

@@ -4,15 +4,18 @@ Go is an open source project.
 
 It is the work of hundreds of contributors. We appreciate your help!
 
+## Before filing an issue
+
+If you are unsure whether you have found a bug, please consider asking in the [golang-nuts mailing
+list](https://groups.google.com/forum/#!forum/golang-nuts) or [other forums](https://golang.org/help/) first. If
+the behavior you are seeing is confirmed as a bug or issue, it can easily be re-raised in the issue tracker.
 
 ## Filing issues
 
-General questions should go to the
-[golang-nuts mailing list](https://groups.google.com/group/golang-nuts) or
-[other forum](https://golang.org/wiki/Questions) instead of the issue tracker.
-The gophers there will answer or ask you to file an issue if you've tripped over a bug.
+Sensitive security-related issues should be reported to [security@golang.org](mailto:security@golang.org).
+See the [security policy](https://golang.org/security) for details.
 
-When filing an issue, make sure to answer these five questions:
+Otherwise, when filing an issue, make sure to answer these five questions:
 
 1. What version of Go are you using (`go version`)?
 2. What operating system and processor architecture are you using?
@@ -22,12 +25,9 @@ When filing an issue, make sure to answer these five questions:
 
 For change proposals, see [Proposing Changes To Go](https://github.com/golang/proposal/).
 
-Sensitive security-related issues should be reported to [security@golang.org](mailto:security@golang.org).
-
 ## Contributing code
 
-Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html)
-before sending patches.
+Please read the [Contribution Guidelines](https://golang.org/doc/contribute.html) before sending patches.
 
 **We do not accept GitHub pull requests**
 (we use [an instance](https://go-review.googlesource.com/) of the