docs: start a developer guide

where we can collection links to specs, note spec deviations etc.

docs/dev/id3.rst

@@ -0,0 +1,37 @@
+ID3
+===
+
+`ID3 Website <https://web.archive.org/web/20190103005542/http://id3.org/Home>`__
+
+ID3v2.4.0:
+    * `ID3v2.4.0 Main Structure <https://web.archive.org/web/20200805181446/https://id3.org/id3v2.4.0-structure>`__
+    * `ID3v2.4.0 Native Frames <https://web.archive.org/web/20200806134515/https://id3.org/id3v2.4.0-frames>`__
+    * `ID3v2.4.0 Changes <https://web.archive.org/web/20181102144806/http://www.id3.org/id3v2.4.0-changes>`__
+
+ID3v2.3.0:
+    * `ID3v2.3.0 Informal standard <https://web.archive.org/web/20180814201738/http://id3.org/d3v2.3.0>`__
+    * `ID3v2 Programming Guidlines <https://web.archive.org/web/20180814205538/http://id3.org/id3guide>`__
+
+ID3v2.2.0:
+    * `ID3v2.2.0 Informal standard <https://web.archive.org/web/20181229090829/http://www.id3.org/id3v2-00>`__
+
+Additional standards:
+  * `ID3v2 Chapter Frame Addendum v1.0 <https://web.archive.org/web/20200809234704/https://id3.org/id3v2-chapters-1.0>`__
+  * `ID3v2 Accessibility Addendum v1.0 <https://web.archive.org/web/20181119131208/http://id3.org/id3v2-accessibility-1.0>`__
+
+
+Mutagen Implementation Details
+------------------------------
+
+* We don't interpret or write the "ID3v2 extended header". An existing extended
+  header gets removed when saving the tags.
+
+  * https://github.com/quodlibet/mutagen/pull/631#issuecomment-1742118078
+
+* We don't support appended tags. They will neither be found nor updated.
+
+  * https://github.com/quodlibet/mutagen/issues/78
+
+* We always NULL-terminate multivalue text frames and ignore them when parsing
+
+  * https://github.com/quodlibet/mutagen/issues/379#issuecomment-486852503

docs/dev/index.rst

@@ -0,0 +1,8 @@
+Developer Guide
+===============
+
+.. toctree::
+    :titlesonly:
+    :maxdepth: 2
+
+    id3

docs/index.rst

@@ -13,6 +13,7 @@
     user/index
     api/index
     man/index
+    dev/index
     contact
 
 .. title:: Overview