CONTRIBUTING.md updates

Author: david-schrenker

CONTRIBUTING.md

@@ -14,7 +14,7 @@ do so.
 
 * Check that the issue has not already been reported.
 * Check that the issue has not already been fixed in the latest code
-  (a.k.a. `master`).
+  (a.k.a. `main`).
 * Be clear, concise and precise in your description of the problem.
 * Open an issue with a descriptive title and a summary in grammatically correct,
   complete sentences.
@@ -38,10 +38,10 @@ consider what kind of change it is:
 
 - Create a personal fork of the project on Github.
 - If you created your fork a while ago be sure to pull upstream changes into your local repository.
-- Create a new branch to work on! Branch from `master`.
+- Create a new branch to work on! Branch from `main`.
 - Implement/fix your feature, comment your code.
 - Write or adapt tests as needed.
 - Add or change the documentation as needed.
 - Squash your commits into a single commit with git's [interactive rebase](https://help.github.com/articles/interactive-rebase). Create a new branch if necessary.
 - Push your branch to your fork on Github, the remote `origin`.
-- From your fork open a pull request with a clear title and description in grammatically correct, complete sentences.
+- From your fork open a pull request with a clear title and description in grammatically correct, complete sentences.