Add some docs

README.md

@@ -6,7 +6,64 @@ Shadowmire syncs PyPI (or plain HTTP(S) PyPI mirrors using Shadowmire) with a li
 
 ### Background
 
-PyPI's XML-RPC Mirroring Support: <https://warehouse.pypa.io/api-reference/xml-rpc.html#mirroring-support>
+Bandersnatch is the recommended solution to sync from PyPI. However, it has these 2 issues that haven't been solved for a long time:
+
+- Bandersnatch does not support removing packages that have been removed from upstream, making it easier to be the target of supply chain attack.
+- The upstream must implement [XML-RPC APIs](https://warehouse.pypa.io/api-reference/xml-rpc.html#mirroring-support), which is not acceptable for most mirror sites.
+
+Shadowmire is a light solution to these issues.
+
+### Syncing Protocol
+
+#### From PyPI
+
+PyPI's XML-RPC APIs have `list_packages_with_serial()` method to list ALL packages with "serial" (you could consider it as a version integer that just increases every few moments). `changelog_last_serial()` and `changelog_since_serial()` are NOT used as they could not handle package deletion. Local packages not in the list result are removed.
+
+Results from `list_packages_with_serial()` are stored in `remote.json`. `local.db` is a sqlite database which just stores every local package name and its local serial. `local.json` is dumped from `local.db` for downstream cosumption.
+
+#### From upstream using shadowmire
+
+Obviously, `list_packages_with_serial()`'s alternative is the `local.json`, which could be easily served by any HTTP server. Don't use `local.db`, as it could have consistency issues when shadowmire upstream is syncing.
+
+### How to use
+
+If you just need to fetch all indexes (and then use a cache solution for packages):
+
+```shell
+REPO=/path/to/pypi ./shadowmire.py sync
+```
+
+If `REPO` env is not set, it defaults to current working directory.
+
+If you need to download all packages, add `--sync-packages`.
+
+```shell
+./shadowmire.py sync --sync-packages
+```
+
+Sync command also supports `--exclude` -- you could give multiple regexes like this:
+
+```shell
+./shadowmire.py sync --exclude package1 --exclude ^0
+```
+
+And `--shadowmire-upstream`, if you don't want to sync from PyPI directly.
+
+```shell
+./shadowmire.py sync --shadowmire-upstream http://example.com/pypi/
+```
+
+If you already have a pypi repo, use `genlocal` first to generate a local db:
+
+```shell
+./shadowmire.py genlocal
+```
+
+Verify command could be used if you believe that something is wrong. It would remove packages NOT in local db, update all local packages, and delete unreferenced files in `packages` folder:
+
+```shell
+./shadowmire.py verify
+```
 
 ## Acknowledgements