Add contributor guide

CONTRIBUTING.md

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+# Contributor Guide
+
+Thank you for investing your time in contributing to our project!
+
+Read our [Code of Conduct](https://scientific-python.org/code_of_conduct/) to keep our community approachable and respectable.
+
+If you haven't already, come find us in [here](https://discuss.scientific-python.org/). We want you working on things you're excited about.
+
+## New contributor guide
+
+Here are some resources to help you get started with open source contributions:
+
+- [Finding ways to contribute to open source on GitHub](https://docs.github.com/en/get-started/exploring-projects-on-github/finding-ways-to-contribute-to-open-source-on-github)
+- [Set up Git](https://docs.github.com/en/get-started/quickstart/set-up-git)
+- [GitHub flow](https://docs.github.com/en/get-started/quickstart/github-flow)
+- [Collaborating with pull requests](https://docs.github.com/en/github/collaborating-with-pull-requests)
+
+## Development Workflow
+
+1.  If you are a first-time contributor:
+
+    -   Go to <https://github.com/scientific-python/scientific-python-hugo-theme/> and click the
+        \"fork\" button to create your own copy of the project.
+
+    -   Clone the project to your local computer:
+
+            git clone git@github.com:scientific-python/scientific-python-hugo-theme.git
+
+    -   Navigate to your `scientific-python-hugo-theme` and add your personal GitHub repository:
+
+            git remote add <your-username> git@github.com:<your-username>/scientific-python-hugo-theme.git
+
+    -   Now, you have remote repositories named:
+
+        -   `origin`, which refers to the `scientific-python/scientific-python-hugo-theme` repository
+        -   `<your-username>`, which refers to your personal fork of `scientific-python/scientific-python-hugo-theme`
+
+2.  Develop your contribution:
+
+    -   Pull the latest changes from upstream:
+
+            git switch main
+            git pull origin main
+
+    -   Create a branch for the feature you want to work on. Since the branch name will appear in the merge message, use a sensible
+        name such as `bugfix-for-issue-1480`:
+
+            git switch -c bugfix-for-issue-1480
+
+    -   Commit locally as you progress (`git add` and `git commit`)
+
+3.  Submit your contribution:
+
+    -   Push your changes back to your fork on GitHub:
+
+            git push <your-username> bugfix-for-issue-1480
+
+    -   Go to GitHub. The new branch will show up with a green Pull Request button---click it.
+
+4.  Review process:
+
+    -   Every Pull Request (PR) update triggers a set of [continuous
+        integration](https://en.wikipedia.org/wiki/Continuous_integration)
+        services that check that the code is up to standards and passes
+        all our tests. These checks must pass before your PR can be
+        merged. If one of the checks fails, you can find out why by
+        clicking on the \"failed\" icon (red cross) and inspecting the
+        build and test log.
+    -   Reviewers (the other developers and interested community
+        members) will write inline and/or general comments on your PR to
+        help you improve its implementation, documentation, and style.
+        Every single developer working on the project has their code
+        reviewed, and we\'ve come to see it as friendly conversation
+        from which we all learn and the overall code quality benefits.
+        Therefore, please don\'t let the review discourage you from
+        contributing: its only aim is to improve the quality of project,
+        not to criticize (we are, after all, very grateful for the time
+        you\'re donating!).
+    -   To update your PR, make your changes on your local repository
+        and commit. As soon as those changes are pushed up (to the same
+        branch as before) the PR will update automatically.
+
+    ::: note
+    ::: title
+    Note
+    :::
+
+    If the PR closes an issue, make sure that GitHub knows to
+    automatically close the issue when the PR is merged. For example, if
+    the PR closes issue number 1480, you could use the phrase \"Fixes
+    #1480\" in the PR description or commit message.
+    :::
+
+## Divergence from `origin main`
+
+If GitHub indicates that the branch of your Pull Request can no longer
+be merged automatically, merge the main branch into yours:
+
+    git fetch origin main
+    git merge origin/main
+
+If any conflicts occur, they need to be fixed before continuing. See
+which files are in conflict using:
+
+    git status
+
+Which displays a message like:
+
+    Unmerged paths:
+      (use "git add <file>..." to mark resolution)
+
+      both modified:   file_with_conflict.txt
+
+Inside the conflicted file, you\'ll find sections like these:
+
+    <<<<<<< HEAD
+    The way the text looks in your branch
+    =======
+    The way the text looks in the main branch
+    >>>>>>> main
+
+Choose one version of the text that should be kept, and delete the rest:
+
+    The way the text looks in your branch
+
+Now, add the fixed file:
+
+    git add file_with_conflict.txt
+
+Once you\'ve fixed all merge conflicts, do:
+
+    git commit
+
+::: note
+::: title
+Note
+:::
+
+Advanced Git users may want to rebase instead of merge, but we squash
+and merge PRs either way.
+:::