Support mirrors within

DEVEL.md

@@ -84,3 +84,31 @@ libraries that are compiled and installed.
   - Patches are licensed under the license of the project which they are
     patching.
   - All files (excluding files within submodules) must comply with REUSE v3.0.
+
+## `sources` file
+
+The format of the sources file for a HTTP source file is:
+
+```
+<url> <checksum> [filename]
+```
+
+For a Git repository snapshot:
+
+```
+<git url> <URL to HTTP snapshot> <checksum> [filename]
+```
+
+The format of a git url must always be `git://<path to repository>~<reference>`.
+`reference` is, for instance, a commit, tag or branch. Always use
+Git protocol path, except for GitHub, in which `git://` is automagically
+changed to `https://`.
+
+The URL to HTTP snapshot may be `_` (a single underscore) if no HTTP snapshot
+exists. In this case, the filename is compulsory. The checksum is of the Git
+snapshot, generated with `git archive`. See `mirror.sh` for more detailed rules.
+
+Some specific helpful things:
+- prefer `.tar.gz`
+- For GitHub, all snpashots are generated with long commit IDs, so use long
+  commit IDs.