Simplify contributing.md; move details into GitHub templates

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

.github/ISSUE_TEMPLATE.md

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+### 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).

.github/PULL_REQUEST_TEMPLATE.md

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+### Links to any relevant github issues:
+
+- [Issue description](http://example.com/org/repo/issues/411). Any other useful information.
+
+### What's in the the PR
+
+- [ ] A description of the changes proposed in the pull request.
+  - [ ] Any areas or issues reviewer should pay attention to? (comments in diff okay).
+- [ ] Update `/docs` to include, whenever possible, a new, suitable recommendation about how to use
+    the feature.
+- [ ] Extra Credit: [Confirm it runs and tests pass on the Rubies specified in the Travis
+  config](.travis.yml). A maintainer will otherwise confirm it runs on these.
+- [ ] *Bonus Points* Update [CHANGELOG.md](https://github.com/rails-api/active_model_serializers/blob/master/CHANGELOG.md)
+  with a brief description of any breaking changes, fixes, features, or
+  miscellaneous changes at the top of the proper version section.
+
+For more general information, see [Submitting a pull request](
+https://github.com/rails-api/active_model_serializers/blob/master/CONTRIBUTING.md#submitting-a-pull-request-pr).