Add a RELEASING document

Removes RUNNING_TESTS file, that linked to CONTRIBUTING.

RELEASING.md

+Releasing paperclip
+
+1. Update `lib/paperclip/version.rb` file accordingly.
+2. Update `NEWS` to reflect the changes since last release.
+3. Commit changes. There shouldn’t be code changes, and thus CI doesn’t need to
+   run, you can then add “[ci skip]” to the commit message.
+4. Tag the release: `git tag vVERSION`
+5. Push changes: `git push --tags`
+6. Build and publish the gem:
+
+   ```bash
+   gem build paperclip.gemspec
+   gem push paperclip-*.gem
+   ```
+
+7. Announce the new release, making sure to say “thank you” to the contributors
+   who helped shape this version.

RUNNING_TESTS.md

-Running Tests
-=============
-
-Please see `CONTRIBUTING.md` in "Running Tests" section for more information.