Add a design document for multiple API support

DESIGN.md

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+# Design notes
+
+This document lists a few details needed for designing the right APIs for the different Danbooru implementations.
+Unfortunately most of these are laregly incompatible for syntax and URL paths.
+
+## Konachan.{com,net} - yande.re
+
+These are the boards originally supported by Danbooru Client.
+
+Supported (doesn't list what Danbooru Client doesn't need at the moment):
+
+- posts
+- pools[^1]
+- tags[^2]
+- search by tags
+- search by related tags
+
+Return types:
+
+- JSON[^3]
+- XML
+
+Structure of a post:
+
+TODO
+
+Structure of a tag:
+
+TODO
+
+Structure of a pool:
+
+TODO
+
+
+## Danbooru (danbooru.donmai.us)
+
+The "original" Danbooru implementation. The API is largely different (mostly REST URL syntax) but functionality is the same, or even better than the Konachan / yande.re Danbooru.
+
+Supported:
+
+- TODO
+
+Return types:
+
+- JSON
+- XML
+
+Structure of a post:
+
+TODO
+
+Structure of a tag:
+
+TODO
+
+Structure of a pool:
+
+TODO
+
+
+
+## Gelbooru
+
+Very limited API. Basically only returns posts.
+
+Supported:
+
+- Posts
+
+Return types:
+
+- XML
+
+Structure of a post:
+
+TODO
+
+Structure of a tag:
+
+TODO
+
+Structure of a pool:
+
+TODO
+
+
+
+[^1]: The service often returns invalid data via JSON, hence it's more reliable via XML
+[^2]: No method for fetching tags in batches
+[^3]: Tags are returned in the wrong order via JSON, so it's better to use XML