First draft of issue and PR templates.

Apparently we can also get contributing.md to be linked from
issues and PRs, but it would also have to be in .github folder
or in root of the project.

.github/pull_request_template.md

@@ -0,0 +1,25 @@
+<!--
+Please do not submit pull requests changing the `version.hpp`
+or the single-include `catch.hpp` file, these are changed
+only when a new release is made.
+-->
+
+
+## Description
+<!--
+Describe the what and the why of your pull request. Remember that these two
+are usually a bit different. As an example, if you have made various changes
+to decrease the number of new strings allocated, thats what. The why probably
+was that you have a large set of tests and found that this speeds them up.
+-->
+
+## GitHub Issues
+<!-- 
+If this PR was motivated by some existing issues, reference them here.
+
+If it is a simple bug-fix, please also add a line like 'Closes #123'
+to your commit message, so that it is automatically closed.
+If it is not, don't, as it might take several iterations for a feature
+to be done properly. If in doubt, leave it open and reference it in the
+PR itself, so that maintainers can decide.
+-->