Add readme to //components/module_installer

Change-Id: If996bf87eb9903ffd92257967c8f63ab2a24a298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1903757
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Fred Mello <fredmello@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713558}

components/module_installer/readme.md

+# Chrome on Android Dynamic Feature Module Installer Backend
+
+This component houses code to install and load Android
+[dynamice feature modules](https://developer.android.com/guide/app-bundle). See
+the [onboarding guide](../../docs/android_dynamic_feature_modules.md) for how to
+create such a feature module in Chrome. Broadly, this component offers two APIs
+- _install engine_ to install modules and _module builder_ to set up modules on
+first access.
+
+## Install Engine
+
+The install engine is a wrapper around Play Core's
+[split install API](https://developer.android.com/guide/app-bundle/playcore)
+that performs extra setup such as
+[SplitCompat](https://developer.android.com/guide/app-bundle/playcore#access_downloaded_modules),
+collects metrics and provides
+[fake install](android/java/src/org/chromium/components/module_installer/engine/FakeEngine.java).
+You can install a module by name with the following code snippet:
+
+```java
+InstallEngine installEngine = new EngineFactory().getEngine();
+installEngine.install("foo", success -> {
+    // Module installed successfully if |success| is true.
+});
+```
+
+You can use the install engine on its own but will have to take care of module
+setup such as loading native code and resources. To simplify that you can use
+the module builder API.
+
+## Module Builder
+
+The module builder simplifies module set up by loading native code and resources
+on first module access and determines whether a module is installed. The module
+builder uses the install engine in the back. It primarily provides the following
+building blocks:
+
+* [`@ModuleInterface`](android/java/src/org/chromium/components/module_installer/builder/ModuleInterface.java)
+  to annotate the entry point of your module. Using this with the
+  [`module_interface_processor`](android/BUILD.gn) will create a module class
+  such as `FooModule` that lets you install and load a module. See
+  [`Module`](android/java/src/org/chromium/components/module_installer/builder/Module.java)
+  for its interface.
+
+* [`Module`](android/java/src/org/chromium/components/module_installer/builder/Module.java)
+  needs to be able to retrieve a
+  [`ModuleDescriptor`](android/java/src/org/chromium/components/module_installer/builder/ModuleDescriptor.java)
+  implementation for each module via reflection. You can create such an
+  implementation with the [`module_desc_java`](android/module_desc_java.gni)
+  template.