Introduce README file for chrome/android/webapk

chrome/android/webapk/README

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+WebAPKs are a proposal to improve homescreen shortcuts by:
+- Giving them a spot in the app launcher
+- Enabling web pages to open a WebAPK when a user taps on a URL which belongs to
+    a WebAPK
+- Improving resource and permission attribution
+
+In short, they attempt to providing additional glue to give Progress Web Apps
+deeper integration into the Android OS, similar to native apps.
+
+The directory is broken down as follows:
+- shell_apk/ - a template APK which can be customized for a given domain based
+    on its Web App Manifest: https://www.w3.org/TR/appmanifest/
+- libs/client/ - a client library meant for a host browser to interact with
+    WebAPKs
+- libs/common/ - common library used by the WebAPK and host browser
+- libs/runtime_library/ - a library which can be dynamically loaded by a
+    ShellAPK. A host browser provides the library which the ShellAPK uses to
+    implement the majority of its functionaly. For chrome, this library is
+    packaged into Chrome's assets file and extracted as needed. This approach
+    ensures that the majority of the WebAPK logic is shared and can be updated
+    as often as the host browser, without the need for updating each individual
+    WebAPK.