Include derive(JsonSchema) docs in rustdoc

docs/_includes/deriving.md

@@ -0,0 +1,28 @@
+# Deriving JsonSchema
+
+The most important trait in Schemars is `JsonSchema`, and the most important function of that trait is `json_schema(...)` which returns a JSON schema describing the type. Implementing this manually on many types would be slow and error-prone, so Schemars includes a derive macro which can implement that trait for you. Any derived implementation of `JsonSchema` should create a schema that describes the JSON representation of the type if it were to be serialized by serde_json.
+
+Usually, all you need to do to use it is to add a `#[derive(JsonSchema)]` attribute to your type:
+
+```rust
+use schemars::{JsonSchema, schema_for};
+
+#[derive(JsonSchema, Debug)]
+struct Point {
+    x: i32,
+    y: i32,
+}
+
+fn main() {
+    let schema = schema_for!(Point);
+
+    let serialized = serde_json::to_string(&schema).unwrap();
+    println!("{}", serialized);
+}
+```
+
+<!-- TODO:
+show example output
+requirements - when can/can't it be derived
+generic params behaviour
+-->