Add note for github users on CONTRIBUTING.md / simplify link code

CONTRIBUTING.md

@@ -2,64 +2,70 @@ Contributing to VTK
 ===================
 
 This page documents at a very high level how to contribute to VTK.
-Please check our [developer instructions][] for a more detailed guide to
-developing and contributing to the project, and our [VTK Git README][]
+Please check our [developer instructions] for a more detailed guide to
+developing and contributing to the project, and our [VTK Git README]
 for additional information.
 
-1.  Register [GitLab Access] to create an account and select a user name.
+1.  The canonical VTK source is maintained on a GitLab instance
+    at https://gitlab.kitware.com/vtk/vtk.<br/>
+    [Create an account] there if you don't have one yet.<br/>
+    **Note:** *If you're reading this document on GitHub,
+    bear in mind that it is just a mirror, so pull requests here aren't merged.
+    But don't worry, the workflow in GitLab is pretty similar
+    and you can even sign up using your GitHub account.*
 
-2.  [Fork VTK][] into your user's namespace on GitLab.
+2.  [Fork VTK] into your user's namespace on GitLab.
 
-3.  Follow the [download instructions][] to create a
+3.  Follow the [download instructions] to create a
     local clone of the main VTK repository:
 
         $ git clone https://gitlab.kitware.com/vtk/vtk.git VTK
         $ cd VTK
     The main repository will be configured as your `origin` remote.
 
-    For more information see: [Setup][]
+    For more information see: [Setup]
 
-4.  Run the [developer setup script][] to prepare your VTK work tree and
+4.  Run the [developer setup script] to prepare your VTK work tree and
     create Git command aliases used below:
 
         $ ./Utilities/SetupForDevelopment.sh
     This will prompt for your GitLab user name and configure a remote
     called `gitlab` to refer to it.
 
-    For more information see: [Setup][]
+    For more information see: [Setup]
 
 5.  Edit files and create commits (repeat as needed):
 
         $ edit file1 file2 file3
         $ git add file1 file2 file3
         $ git commit
 
-    For more information see: [Create a Topic][]
+    For more information see: [Create a Topic]
 
 6.  Push commits in your topic branch to your fork in GitLab:
 
         $ git gitlab-push
 
-    For more information see: [Share a Topic][]
+    For more information see: [Share a Topic]
 
 7.  Visit your fork in GitLab, browse to the "**Merge Requests**" link on the
     left, and use the "**New Merge Request**" button in the upper right to
     create a Merge Request.
 
-    For more information see: [Create a Merge Request][]
+    For more information see: [Create a Merge Request]
 
 
 VTK uses GitLab for code review and Buildbot to test proposed
 patches before they are merged.
 
-Our [Wiki][] is used to document features, flesh out designs and host other
-documentation. Our API is documented using [Doxygen][] with updated
-documentation generated nightly. We have several [Mailing Lists][]
+Our [Wiki] is used to document features, flesh out designs and host other
+documentation. Our API is documented using [Doxygen] with updated
+documentation generated nightly. We have several [Mailing Lists]
 to coordinate development and to provide support.
 
 [VTK Git README]: Documentation/dev/git/README.md
 [developer instructions]: Documentation/dev/git/develop.md
-[GitLab Access]: https://gitlab.kitware.com/users/sign_in
+[Create an account]: https://gitlab.kitware.com/users/sign_in
 [Fork VTK]: https://gitlab.kitware.com/vtk/vtk/fork/new
 [download instructions]: Documentation/dev/git/download.md#clone
 [developer setup script]: /Utilities/SetupForDevelopment.sh