Update pull request creation guidelines

Author: alco

CONTRIBUTING.md

@@ -123,11 +123,12 @@ in order to craft an excellent pull request:
 2. If you cloned a while ago, get the latest changes from upstream:
 
    ```bash
-   git checkout <dev-branch>
-   git pull upstream <dev-branch>
+   git checkout master
+   git pull upstream master
    ```
 
-3. Create a new topic branch (off master) to contain your feature, change, or fix:
+3. Create a new topic branch (off of `master`) to contain your feature, change,
+   or fix:
 
    ```bash
    git checkout -b <topic-branch-name>
@@ -139,25 +140,35 @@ in order to craft an excellent pull request:
    [interactive rebase](https://help.github.com/articles/interactive-rebase)
    feature to tidy up your commits before making them public.
 
-5. Locally merge (or rebase) the upstream development branch into your topic branch:
+5. Push your topic branch up to your fork:
 
    ```bash
-   git pull [--rebase] upstream <dev-branch>
+   git push origin <topic-branch-name>
    ```
 
-6. Push your topic branch up to your fork:
+6. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
+    with a clear title and description.
+
+7. If you haven't updated your pull request for a while, you should consider
+   rebasing on master and resolving any conflicts.
+
+   **IMPORTANT**: _Never ever_ merge upstream `master` into your branches. You
+   should always `git rebase` on `master` to make your changes up to date when
+   necessary.
 
    ```bash
-   git push origin <topic-branch-name>
+   git checkout master
+   git pull upstream master
+   git checkout <your-topic-branch>
+   git rebase master
    ```
 
-7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/)
-    with a clear title and description.
-
 We have saved some excellent pull requests we have received in the past in case
 you are looking for some examples:
 
 * https://github.com/elixir-lang/elixir/pull/992
 * https://github.com/elixir-lang/elixir/pull/1041
 * https://github.com/elixir-lang/elixir/pull/1058
 * https://github.com/elixir-lang/elixir/pull/1059
+
+Thanks for your contributions!