Improve documentation on contributing [skip ci]

Author: ashmaroli

README.md

@@ -101,7 +101,21 @@ This will also auto add:
 
 1. Fork it ( http://github.com/jekyll/jekyll-compose/fork )
 2. Create your feature branch (`git checkout -b my-new-feature`)
-3. Run the specs (`script/cibuild`)
+3. Run the specs and our linter (`script/cibuild`)
 4. Commit your changes (`git commit -am 'Add some feature'`)
 5. Push to the branch (`git push origin my-new-feature`)
 6. Create new Pull Request
+
+### Submitting a Pull Request based on an existing proposal
+
+When submitting a pull request that uses code from an unmerged pull request, please be aware of the following:
+  * Changes proposed in the older pull request is still the original author's property. Moving forward from where they left it
+    means that you're a `co-author`.
+  * GitHub allows attributing
+    [credit to multiple authors](https://help.github.com/en/articles/creating-a-commit-with-multiple-authors)
+    However, pull requests in this project are automatically squashed and then merged onto the base branch. So, only authors and
+    co-authors of the opening commit gets credit once the pull request gets merged.
+  * If the original pull request contained multiple commits, you may squash them into a single commit but ensure that you list
+    any additional authors (and yourselves) as co-authors of that commit.
+  * Use appropriate [keywords](https://help.github.com/en/articles/closing-issues-using-keywords) in your pull request post to
+    link to the existing pull request or issue-ticket so that they're automatically closed when your pull request gets merged.