Update contribution guide
Description
Our GitLab instance has now been opened to the public. Everybody is allowed to create an account immediately. However, people who are not members of our research group will receive an External User account only. The contribution guide in CONTRIBUTING.md should reflect that.
Proposal
Update the "GitLab Account" section of the contribution guide and invite possible contributors to create an account and join the DORiE project.
How to test the implementation?
Nothing to test.
Related issues
None.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information