Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# How to contribute
+
+We happily review/accept third-party Pull Requests. To help get your patches adopted into our plugins, there's a few bits of info that are worth knowing.
+
+## Prerequisites
+
+* Docker
+* PHP 7.4+ at the system level
+
+## Build steps
+
+Wooo! There aren't any! We do suggest adding `slic` to your `$PATH`, though. To do that, place the following in your `.bashrc` or `.zshrc` (or whatever) file:
+
+```bash
+export PATH=$PATH:path/to/slic
+```
+
+## Releases
+
+When we prep a release, we follow these steps:
+
+1. Create a branch with the version number of the release, e.g. `1.0.0`
+2. Merge changes into that branch. This will trigger the Docker images to build.
+3. Ensure that the `CLI_VERSION` has been updated in `slic.php` to reflect the version number of the release.
+4. Ensure that the `changelog.md` file has been updated.
+5. Merge the release branch into `main`.
+6. Delete the release branch.
+7. Tag the release with the changelog.md contents for that release. This will trigger the final Docker images to build.