Add more documentation on how to write an extension (#1145)

CONTRIBUTING.md

@@ -12,6 +12,8 @@ Before you start, please note that the ability to use following technologies is
 
 The quickest way to get started is to copy an existing extension's folder structure and renaming it as needed. Of course, that also means that there's plenty of existing extensions that you can reference as you go!
 
+### Setting up a module
+
 Make sure that your new extension's `build.gradle` file follows the following structure:
 
 ```gradle
@@ -41,6 +43,60 @@ The catalogue's version name is based off of `libVersion` and `extVersionCode`.
 
 \* Note: this library only contains the method definitions so that the compiler can resolve them. The actual implementation is written in Tachiyomi.
 
+### Additional dependencies
+
+You may find yourself needing additional functionality and wanting to add more dependencies to your `build.gradle` file. Since extensions are run within the main Tachiyomi app, you can make use of [its dependencies](https://github.com/inorichi/tachiyomi/blob/master/app/build.gradle).
+
+For example, an extension that needs Gson could add the following:
+
+```
+dependencies {
+    compileOnly 'com.google.code.gson:gson:2.8.2'
+}
+```
+
+Notice that we're using `compileOnly` instead of `implementation`, since the app already contains it. You could use `implementation` instead, if it's a new dependency, or you prefer not to rely on whatever the main app has (at the expensive of app size).
+
+### Core stubs and libraries
+
+#### Extensions library
+
+Extensions rely on stubs defined in [tachiyomi-extensions-lib](https://github.com/inorichi/tachiyomi-extensions-lib), which simply provides some interfaces for compiling extensions. These interfaces match what's found in the main Tachiyomi app. The exact version used is configured with `libVersion`. The latest version should be preferred.
+
+#### Preference stub
+
+[`preference-stub`](https://github.com/inorichi/tachiyomi-extensions/tree/master/lib/preference-stub) provides the [`ConfigurableSource` interface](https://github.com/inorichi/tachiyomi-extensions/blob/master/lib/preference-stub/src/main/java/eu/kanade/tachiyomi/source/ConfigurableSource.java) for extensions, as well as stubs for Android preferences.
+
+```
+dependencies {
+    compileOnly project(':preference-stub')
+}
+```
+
+#### Duktape stub
+
+[`duktape-stub`](https://github.com/inorichi/tachiyomi-extensions/tree/master/lib/duktape-stub) provides stubs for using Duktape functionality without pulling in the full library. Functionality is bundled into the main Tachiyomi app.
+
+```
+dependencies {
+    compileOnly project(':duktape-stub')
+}
+```
+
+#### Rate limiting library
+
+[`lib-ratelimit`](https://github.com/inorichi/tachiyomi-extensions/tree/master/lib/ratelimit) is a library for adding rate limiting functionality.
+
+```
+dependencies {
+    implementation project(':lib-ratelimit')
+}
+```
+
+### Useful knowledge
+
+An extension should at least extend the [`ParsedHttpSource`](https://github.com/inorichi/tachiyomi-extensions-lib/blob/master/library/src/main/java/eu/kanade/tachiyomi/source/online/ParsedHttpSource.kt) class.
+
 
 ## Running