fix: add bump-version instruction (#16)

Author: savka2017

CONTRIBUTING.md

@@ -32,5 +32,13 @@ Some things that will increase the chance that your pull request is accepted:
 * Write tests.
 * Write a [good commit message][commit], using also [release-please approach][release-please]
 
+## How to create new release version
+
+1. Make changes to code (make sure to create correct commits with [release-please approach][release-please])
+2. Update version in `lib/json_logic/version.rb`
+3. Create PR with changes
+4. After PR is merged, release-please will create new PR 
+5. Merge the final PR
+
 [commit]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
 [release-please]: https://github.com/googleapis/release-please