docs(views): extend README for josh-filter usage

Also document how these can be cloned by referring to a custom Gerrit
ref.

Change-Id: I5495a1c29bdd6d78215af7307953437f9cbca5db
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8198
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

views/README.md

@@ -3,4 +3,29 @@ depot views
 
 This folder contains external views of depot content, defined using
 josh workspaces. See the individual views for a description of their
-content and usage information.
+individual content and usage information.
+
+Testing changes locally
+-----------------------
+
+Generally, when iterating on these files, it's best to locally invoke `josh-
+filter` (from `//third_party//josh`) locally to inspect how the workspace would
+look like:
+
+  - Commit your changes. This is required, as `josh-filter` operates on your
+    `HEAD`, not working directory state.
+  - Invoke `josh-filter` with the filter expression,
+    for example `josh-filter ':workspace=views/kit'`.
+  - Peek at the synthesized git history by looking at `FILTERED_HEAD`.
+
+Testing changes in Gerrit
+-------------------------
+
+It's also possible to clone resulting workspaces for CLs that were already
+pushed to Gerrit, but didn't land in master yet.
+
+For CL1234 at revision 2, the URL passed to `git clone` would look like this:
+
+```
+https://code.tvl.fyi/depot.git@refs/changes/32/1234/2:workspace=views/kit.git
+````