localstorage: add readme

localstorage/readme.md

@@ -0,0 +1,9 @@
+# Localstorage
+
+This represents the working directory where `cas` is run.  its purpose is to read artifacts from disk, and send to a backend, and also to write the files from the backend to disk.
+
+## `.archive` files
+
+If `ReadFile()` is called with `.archive`, the working directory of the file is compressed into a tar file, and then the resulting archive is returned.
+
+If `WriteFile()` is called with `.archive`, the file is extracted into the directory instead of the `.archive` file being written to disk itself.