Update README.md with caching info

README.md

@@ -8,6 +8,9 @@ This enables your keyboard's standard media keys to control MPD, as well as more
 The recommended way to install mpd-now-playable and its dependencies is with [pipx](https://pypa.github.io/pipx/):
 ```shell
 pipx install mpd-now-playable
+# or, if you'd like to use a separate cache service, one of these:
+pipx install mpd-now-playable[redis,msgpack]
+pipx install mpd-now-playable[memcached,msgpack]
 ```
 
 Once pipx is done, the `mpd-now-playable` script should be available on your `$PATH` and ready to use.
@@ -18,7 +21,7 @@ Most likely, you'll want mpd-now-playable to stay running in the background as a
 
 You may not need any configuration! If you've got a relatively normal MPD setup on your local machine, mpd-now-playable ought to just work out of the box, as it uses sensible defaults. If you need to control a remote MPD server, or your MPD clients use a password, though, you'll need configuration for that use case.
 
-Currently, mpd-now-playable can only be configured through environment variables. Command-line arguments are intentionally not supported, since your MPD password is among the supported settings and command-line arguments are not a secure way to pass secrets such as passwords into commands. Reading configuration from a file is secure, so mpd-now-playable may support a config file in future.
+Currently, mpd-now-playable can only be configured through environment variables. Command-line arguments are intentionally not supported, since your MPD password is among the supported settings and command-line arguments are not a secure way to pass secrets such as passwords into commands. Reading configuration from a file is secure, provided the file itself is kept secure, so mpd-now-playable may support a config file in future.
 
 The following environment variables are read. The `MPD_HOST` and `MPD_PORT` variables are supported in the same way `mpc` uses them, but you can alternatively provide your password as a separate `MPD_PASSWORD` variable if you wish.
 
@@ -26,12 +29,20 @@ The following environment variables are read. The `MPD_HOST` and `MPD_PORT` vari
 - `MPD_PORT` - defaults to 6600, which will almost always be the correct port to use.
 - `MPD_PASSWORD` - has no default. Set this only if your MPD server expects a password. You can also provide a password by setting `MPD_HOST=password@host`, if you want to be consistent with how `mpc` works.
 
+Additionally, mpd-now-playable caches your album artwork, by default simply in memory. It may be configured to use an external cache, and currently supports Redis and Memcached for this purpose. To use one of these, set the environment variable `MPD_NOW_PLAYABLE_CACHE` to an appropriate URL for your cache service:
+
+- For Redis, use something like `redis://localhost:6379/0`.
+- For Memcached, use something like `memcached://localhost:11211`.
+
+You may provide a `namespace` query parameter to prefix cache keys if you wish, as well as a `password` query parameter if your service requires a password to access. As with your other environment variables, keep your cache password secure.
+
 One simple secure way to set your environment variables is with a small wrapper script like this:
 ```shell
 #!/bin/sh
 export MPD_HOSTNAME=my.cool.mpd.host
 export MPD_PORT=6700
 export MPD_PASSWORD=swordfish
+export MPD_NOW_PLAYABLE_CACHE='redis://localhost:6379/0?namespace=mpd-now-playable&password=fishsword'
 exec mpd-now-playable
 ```
 Make sure this wrapper script is only readable by you, with something like `chmod 700`!