Add README to document the communication with the server

While at it, move README-STUDIO.txt into the newly created file.

Change-Id: I3e853a32d57f2d2bea3c41f7e8ec6fb22859e5f5

README-STUDIO.txt

@@ -1,14 +0,0 @@
-How to build with Android Studio
-================================
-
-Updater needs access to the system API, therefore it can't be built only using
-the public SDK. You first need to generate the libraries with all the needed
-classes. The application also needs elevated privileges, so you need to sign
-it with the right key to update the one in the system partition. To do this:
-
- - Generate a keystore and keystore.properties using gen-keystore.sh
- - Build the dependencies running 'make UpdaterStudio'. This command will add
-   the needed libraries in system_libraries/.
-
-You need to do the above once, unless Android Studio can't find some symbol.
-In this case, rebuild the system libraries with 'make UpdaterStudio'.

README.md

@@ -0,0 +1,50 @@
+Updater
+=======
+Simple application to download and apply OTA packages.
+
+
+Server requirements
+-------------------
+The app sends `GET` requests to the URL defined by the `updater_server_url`
+resource (or the `lineage.updater.uri` system property) and expects as response
+a JSON with the following structure:
+```json
+{
+  "response": [
+    {
+      "datetime": 1230764400,
+      "filename": "ota-package.zip",
+      "id": "5eb63bbbe01eeed093cb22bb8f5acdc3",
+      "romtype": "nightly",
+      "url": "https://example.com/ota-package.zip",
+      "version": "15.1"
+    }
+  ]
+}
+```
+
+The `datetime` attribute is the build date expressed as UNIX timestamp.  
+The `filename` attribute is the name of the file to be downloaded.  
+The `id` attribute is a string that uniquely identifies the update.  
+The `romtype` attribute is the string to be compared with the `ro.lineage.releasetype` property.  
+The `url` attribute is the URL of the file to be downloaded.  
+The `version` attribute is the string to be compared with the `ro.lineage.build.version` property.  
+
+Additional attributes are ignored.
+
+
+Build with Android Studio
+-------------------------
+Updater needs access to the system API, therefore it can't be built only using
+the public SDK. You first need to generate the libraries with all the needed
+classes. The application also needs elevated privileges, so you need to sign
+it with the right key to update the one in the system partition. To do this:
+
+ - Place this directory anywhere in the Android source tree
+ - Generate a keystore and keystore.properties using `gen-keystore.sh`
+ - Build the dependencies running `make UpdaterStudio` from the root of the
+   Android source tree. This command will add the needed libraries in
+   `system_libraries/`.
+
+You need to do the above once, unless Android Studio can't find some symbol.
+In this case, rebuild the system libraries with `make UpdaterStudio`.