Add simple usage docs to each adapter.

retrofit-adapters/guava/README.md

@@ -4,6 +4,26 @@ Guava Adapter
 An `Adapter` for adapting [Guava][1] `ListenableFuture`.
 
 
+Usage
+-----
+
+Add `GuavaCallAdapterFactory` as a `Call` adapter when building your `Retrofit` instance:
+```java
+Retrofit retrofit = new Retrofit.Builder()
+    .baseUrl("https://example.com/")
+    .addCallAdapterFactory(GuavaCallAdapterFactory.create())
+    .build();
+```
+
+Your service methods can now use `ListenableFuture` as their return type.
+```java
+interface MyService {
+  @GET("/user")
+  ListenableFuture<User> getUser();
+}
+```
+
+
 Download
 --------
 

retrofit-adapters/java8/README.md

@@ -4,6 +4,26 @@ Java8 Adapter
 An `Adapter` for adapting [Java8][1] `CompletableFuture`.
 
 
+Usage
+-----
+
+Add `Java8CallAdapterFactory` as a `Call` adapter when building your `Retrofit` instance:
+```java
+Retrofit retrofit = new Retrofit.Builder()
+    .baseUrl("https://example.com/")
+    .addCallAdapterFactory(Java8CallAdapterFactory.create())
+    .build();
+```
+
+Your service methods can now use `CompletableFuture` as their return type.
+```java
+interface MyService {
+  @GET("/user")
+  CompletableFuture<User> getUser();
+}
+```
+
+
 Download
 --------
 

retrofit-adapters/rxjava/README.md

@@ -9,6 +9,34 @@ Available types:
  * `Single<T>`, `Single<Response<T>>`, and `Single<Result<T>>`  where `T` is the body type.
  * `Completable` where response bodies are discarded.
 
+
+Usage
+-----
+
+Add `RxJavaCallAdapterFactory` as a `Call` adapter when building your `Retrofit` instance:
+```java
+Retrofit retrofit = new Retrofit.Builder()
+    .baseUrl("https://example.com/")
+    .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
+    .build();
+```
+
+Your service methods can now use any of the above types as their return type.
+```java
+interface MyService {
+  @GET("/user")
+  Observable<User> getUser();
+}
+```
+
+By default all reactive types execute their requests synchronously. There are multiple ways to
+control the threading on which a request occurs:
+
+ * Call `subscribeOn` on the returned reactive type with a `Scheduler` of your choice.
+ * Use `createAsync()` when creating the factory which will use OkHttp's internal thread pool.
+ * Use `createWithScheduler(Scheduler)` to supply a default subscription `Scheduler`.
+
+
 Download
 --------
 

retrofit-adapters/rxjava2/README.md

@@ -11,6 +11,33 @@ Available types:
  * `Maybe<T>`, `Maybe<Response<T>>`, and `Maybe<Result<T>>`  where `T` is the body type.
  * `Completable` where response bodies are discarded.
 
+
+Usage
+-----
+
+Add `RxJava2CallAdapterFactory` as a `Call` adapter when building your `Retrofit` instance:
+```java
+Retrofit retrofit = new Retrofit.Builder()
+    .baseUrl("https://example.com/")
+    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
+    .build();
+```
+
+Your service methods can now use any of the above types as their return type.
+```java
+interface MyService {
+  @GET("/user")
+  Observable<User> getUser();
+}
+```
+
+By default all reactive types execute their requests synchronously. There are multiple ways to
+control the threading on which a request occurs:
+
+ * Call `subscribeOn` on the returned reactive type with a `Scheduler` of your choice.
+ * Use `createAsync()` when creating the factory which will use OkHttp's internal thread pool.
+ * Use `createWithScheduler(Scheduler)` to supply a default subscription `Scheduler`.
+
 Download
 --------