docs: move markdown docs from .github/ to docs/

The GitHub guide on contributing file says: "Decide whether to store your
contributing guidelines in your repository's root, docs, or .github directory."

https://help.github.com/articles/setting-guidelines-for-repository-contributors/#adding-a-contributing-file

But there's really no advantage to keeping it in the hidden .github/, since
these are public and really belong together with the other documentation.

We can still keep the issue templates under .github/, since they are not really
documentation on their own.

Updated the links pointing to CONTRIBUTING.md to refer to the one in docs/.

docs/RELEASE.md

@@ -0,0 +1,12 @@
+# Steps to a successful release
+
+1. Add all items to NEWS
+2. Update the contributors list in NEWS ("make git-contrib")
+3. Update the time and place in NEWS
+4. Update version in configure.ac and library numbers in Makefile.am
+5. Check that "make distcheck" works
+6. Tag the release ("make git-tag")
+7. Upload the documentation ("make doc-sync")
+8. Close the github milestone and open a new one (https://github.com/systemd/systemd/milestones)
+9. Send announcement to systemd-devel, with a copy&paste from NEWS
+10. Update IRC topic ("/msg chanserv TOPIC #systemd Version NNN released")