Create CONTRIBUTING.md

Author: ivan-hc

CONTRIBUTING.md

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+# Premise
+
+The purpose of this catalog is closely related to the distribution of portable apps and AppImages via the AM package manager. To add an app, open an issue or a pull request at https://github.com/ivan-hc/AM
+
+This catalog is updated every half hour and is based entirely on this list: https://github.com/ivan-hc/AM/blob/main/programs/x86_64-apps
+
+If an app is added to this list, it means that an installation script has also been added here: https://github.com/ivan-hc/AM/tree/main/programs/x86_64
+
+## If an app is **added** to AM
+  - All lists will be automatically updated via https://github.com/ivan-hc/AM/blob/main/tools/am2pla-site
+  - A new .md page will be created. It will use the description in that list and the URL specified in the installation script ("SITE" variable).
+  - A new generic icon will be created using https://github.com/Portable-Linux-Apps/Portable-Linux-Apps.github.io/blob/main/no-icon.png
+
+## If an app is **removed** from AM
+  - All references to it in the lists will be removed
+  - The .md page will be removed
+  - The icon will be removed
+
+## This project is powered by github actions, so...
+**...any attempt to add apps not listed in AM will be automatically overwritten!**
+
+However, you are allowed to add the correct icons and edit the pages of existing apps.
+
+------------------------------------
+
+# Contributing
+
+This catalog is for everyone, so anyone can make a [pull request](https://github.com/Portable-Linux-Apps/Portable-Linux-Apps.github.io/pulls).
+
+## Add icons
+  - Icons must be in PNG format and have the name of the referenced app (for example, the app `tizio` will have the icon `tizio.png`, all lowercase)
+  - Icons must be 128x128 or smaller
+
+## Update app's pages
+  - Write whatever you want, but leave the header (first line, the one starting with #) as is.
+  - You can add buttons to the ones at the bottom, including a donation link.
+  - You can add screenshots or GIFs.
+
+------------------------------------
+
+# Conclusion
+
+If you are able to open an issue here, it means you have a GitHub account, and therefore you can make a pull request yourself.
+
+**You are strongly encouraged to actively participate in this project!**