docs: Document examples (#1567)

Document the process for running the examples. This was fairly
non-obvious and I spent a fair bit of time trying and failing to get
a client credentials token to work before finding in some discussion
that this is known not to work.

The wiki mentions using the oauth example as a way to get a token
and I think that is likely to be the best way to get a token for the
purposes of running the examples too.

Author: SapiensAnatis

examples/README.md

@@ -0,0 +1,36 @@
+# Examples
+
+This folder contains examples of how to use the `librespot` library for various purposes.
+
+## How to run the examples
+
+In general, to invoke an example, clone down the repo and use `cargo` as follows:
+
+```
+cargo run --example [filename]
+```
+
+in which `filename` is the file name of the example, for instance `get_token` or `play`.
+
+### Acquiring an access token
+
+Most examples require an access token as the first positional argument. **Note that an access token
+gained by the client credentials flow will not work**. `librespot-oauth` provides a utility to 
+acquire an access token using an OAuth flow, which will be able to run the examples. To invoke this, 
+run:
+
+```
+cargo run --package librespot-oauth --example oauth_sync
+```
+
+A browser window will open and prompt you to authorize with Spotify. Once done, take the 
+`access_token` property from the dumped object response and proceed to use it in examples. You may
+find it convenient to save it in a shell variable like `$ACCESS_TOKEN`.
+
+Once you have obtained the token you can proceed to run the example. Check each individual
+file to see what arguments are expected. As a demonstration, here is how to invoke the `play` 
+example to play a song -- the second argument is the URI of the track to play.
+
+```
+cargo run --example play "$ACCESS_TOKEN" 2WUy2Uywcj5cP0IXQagO3z
+```