Add CONTRIBUTING.md

Added guidelines for contributing summaries including updating existing summaries and adding new summaries.

.github/CONTRIBUTING.md

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+# Contributing
+## Updating existing summaries
+- Do replace the helper files
+- Compile before uploading such that the PDFs are up-to-date
+- (Optional) Add your name to the credits for the summary
+- Open a pull request
+
+## Adding a new summary
+- include all the `.tex` files that are needed to compile the latex document.
+- include any images or other files that are necessary to compile
+- Compile the document
+- (Optional) Configure auto-upload. To add a secret, please let us know that you want to do that in the PR and you will be given access to add secrets
+
+## What we will accept
+- Summaries written with my helpers / styling
+- Summaries written using your own styling
+- Summaries for electives in the field of CS
+- Summaries for all subjects of BSc and MSc Computer Science at ETHZ
+
+## What will not be accepted
+- Summaries with many obvious errors
+- Summaries not written in any TeX distribution or Typst
+- Summaries that do not compile on our end
+- Summaries for subjects that are not offered at ETHZ in BSc and MSc Computer Science
+- Summaries not following common design practices for LaTeX
+- Summaries for GESS subjects