List the development models at the top

Added an unordered list summarizing the collaborative development models.

Author: Baalkikhaal

content/github/collaborating-with-issues-and-pull-requests/about-collaborative-development-models.md

@@ -12,7 +12,12 @@ topics:
   - pull requests
 ---
 
-There are two main types of development models with which you'd use pull requests. In the *fork and pull model*, anyone can fork an existing repository and push changes to their personal fork. You do not need permission to the source repository to push to a user-owned fork. The changes can be pulled into the source repository by the project maintainer. When you open a pull request proposing changes from your user-owned fork to a branch in the source (upstream) repository, you can allow anyone with push access to the upstream repository to make changes to your pull request.  This model is popular with open source projects as it reduces the amount of friction for new contributors and allows people to work independently without upfront coordination.
+There are two main types of collaborative development models with which you'd use pull requests.
+
+- Fork and pull model
+- Shared repository model
+
+In the *fork and pull model*, anyone can fork an existing repository and push changes to their personal fork. You do not need permission to the source repository to push to a user-owned fork. The changes can be pulled into the source repository by the project maintainer. When you open a pull request proposing changes from your user-owned fork to a branch in the source (upstream) repository, you can allow anyone with push access to the upstream repository to make changes to your pull request.  This model is popular with open source projects as it reduces the amount of friction for new contributors and allows people to work independently without upfront coordination.
 
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