Create CONTRIBUTING.md

CONTRIBUTING.md

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+Generic archive information:
+
+```json
+{
+  "uid": 1234,
+  "name": "Archive Name",
+  "domain": "example.com",
+  "http": true,
+  "https": true,
+  "software": "foolfuuka",
+  "boards": ["a"],
+  "files": ["a"]
+}
+```
+
+Most entries should be self-explanatory.
+
+`uid` stands for "unique id", used on the client-side to save archive-specific settings.
+The `uid` for a new archive should be an increment of the highest one,
+make sure it does not overlap with an archive that previously died for example.
+
+`software` can be `"foolfuuka"` or `"fuuka"` for example.
+
+`files` is the list of boards whose files (images, videos, pdfs) are also archived, even temporarily, and not just thumbnails.