Add details to CONTRIBUTING.md (#169)

* Update CONTRIBUTING.md file

* Stylistic/grammatical changes to CONTRIBUTING

It looks like repository owners might be able to commit back to the PR
branch, so I'm giving it a try.

This commit implements the changes requested in #169. After this, I
think it's ready to be merged!

Author: khwilo

.github/CONTRIBUTING.md

@@ -6,3 +6,38 @@ least one other person on the team needs to look over a PR before merging. The
 purpose of this review requirement is to ensure shared knowledge of the app and
 its changes and to take advantage of the benefits of working together changes
 without any single person being a bottleneck to making progress.
+
+## Steps
+
+1. Fork this repository.
+
+2. Clone the repository by running this command:
+    ```shell
+    $ git clone https://github.com/YOUR-GITHUB-USERNAME/playtime.git
+    ```
+
+3. Add a remote upstream that points to the original Playtime repository:
+    ```shell
+    $ git remote add upstream https://github.com/rubyforgood/playtime.git
+    ```
+
+4. Create a new branch to make changes. The branch name should be relevant to
+   your change (ex. `rubocop` or `setup-mailers`). You can create a new branch
+   with:
+    ```shell
+    $ git checkout -b BRANCH-NAME
+    ```
+
+5. Make your modification, make sure the tests pass, and commit the changes
+   locally:
+    ```shell
+    $ bundle exec rake
+    $ git commit
+    ```
+
+6. Push your new branch to GitHub:
+    ```shell
+    $ git push origin BRANCH-NAME
+    ```
+
+7. Open a pull request on GitHub.