doc: add quick cache readme

doc/CACHE

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+Gitian includes two caches where build-assets may be saved for use with
+subsequent builds.
+
+Common cache:
+All descriptors share this cache. It can be useful for storing fetched sources,
+sharing build assets between descriptors, etc.
+To add or update files, copy them to ~/cache/common from the build script.
+
+Per-descriptor cache
+Files installed to this cache can only be seen by this descriptor. Use this to
+store assets created as part of the build process, to avoid having to rebuild
+them in future builds.
+To add or update files, copy them to ~/cache/$NAME from the build script, where
+$NAME is the value of the descriptor's "name" key.
+
+Before each build, all files and folders in the cache directories will be
+transferred to the VM. After each successful build, the caches will be
+transferred back to the builder (overwriting existing files as necessary).
+
+It is entirely up to the user and the descriptors to verify that cached files
+are actually suitable for re-use. Overwriting existing cached files is _highly_
+discouraged, as that would likely lead to non-deterministic results. Unique
+filenames should always be used.