Simplify contributing.md; move details into GitHub templates

per https://github.com/blog/2111-issue-and-pull-request-templates

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,31 @@
+### Expected behavior and actual behavior.
+
+- What did you expect to happen?
+- What happened? Include as much information as possible, such as:
+  - Nature of reported defect (e.g. user name missing, not "It doesn't work."). Is it intermittent?
+  - The best help here is a failing test. Even better if it's a PR.
+  - Then the steps to reproduce and/or a gist or repository that demonstrates the defect.
+  - Then examples of the code you were using.
+  - Any error messages (including stacktrace, i.e. "Show me the error.")
+
+### Steps to reproduce the issue (i.e. "Show me how to show myself." )
+
+- What were you doing? Include code if possible, such as:
+  - Command line parameters used, if any.
+  - RubyGems code in your Gemfile, if any. Gemfile.lock, if possible.
+  - Any configuration you've made.
+- Things you've tried.
+- Link to source code, if available.
+
+### Specifications
+
+- ActiveModelSerializers version (0.8.x, 0.9.x, 0.10.x, commit ref).
+- What are you using ActiveModelSerializers with? Rails? Grape? Other? Which versions?
+- If you are not running the latest version (please check), and you cannot update it,
+  please specify in your report why you can't update to the latest version.
+- Ruby version with patch level.  And whether you're using rvm, rbenv, etc.
+  - Include `ruby -e "puts RUBY_DESCRIPTION"`.
+- Operating system type + version. e.g. Mac OSX Snow Leopard
+
+For more general guidelines, see [Filing an
+issue](https://github.com/rails-api/active_model_serializers/blob/master/CONTRIBUTING.md#filing-an-issue).