Add documentation for using Lemmy API from Rust (#2639)

diff --git a/crates/api_common/README.md b/crates/api_common/README.md
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+lemmy_api_common
+===
+
+This crate provides all the data types which are necessary to build a client for [Lemmy](https://join-lemmy.org/). You can use them with the HTTP client of your choice.
+
+Here is an example using [reqwest](https://crates.io/crates/reqwest):
+
+```rust
+    let params = GetPosts {
+        community_name: Some("asklemmy".to_string()),
+        ..Default::default()
+    };
+    let client = Client::new();
+    let response = client
+        .get("https://lemmy.ml/api/v3/post/list")
+        .query(&params)
+        .send()
+        .await?;
+    let json = response.json::<GetPostsResponse>().await.unwrap();
+    print!("{:?}", &json);
+```
+
+As you can see, each API endpoint needs a parameter type ( GetPosts), path (/post/list) and response type (GetPostsResponse). You can find the paths and parameter types from [this file](https://github.com/LemmyNet/lemmy/blob/main/src/api_routes_http.rs). For the response types you need to look through the crates [lemmy_api](https://github.com/LemmyNet/lemmy/tree/main/crates/api/src) and [lemmy_api_crud](https://github.com/LemmyNet/lemmy/tree/main/crates/api_crud/src) for the place where Perform/PerformCrud is implemented for the parameter type. The response type is specified as a type parameter on the trait.
+
+For a real example of a Lemmy API client, look at [lemmyBB](https://github.com/LemmyNet/lemmyBB/tree/main/src/api).
+
+Lemmy also provides a websocket API. You can find the full websocket code in [this file](https://github.com/LemmyNet/lemmy/blob/main/src/api_routes_websocket.rs).