add setup instructions/command to load secrets file for Android

Makefile

@@ -8,6 +8,9 @@ apple_extension_gen:
 	$(MAKE) -C apple extension_gen
 
 droid:
+	@if ! [ -e android/Omnivore/app/src/main/res/values/secrets.xml ]; then \
+		cp android/Omnivore/secrets.xml android/Omnivore/app/src/main/res/values/secrets.xml; \
+	fi
 	studio android/Omnivore
 
 webview_gen:

android/Omnivore/README.md

@@ -0,0 +1,11 @@
+# Omnivore - Android
+
+## Setup
+
+From the root directory run the following command:
+
+`make droid`
+
+This will copy a secrets.xml file into the strings directory.
+If you have a PSPDF license key then paste that value into the
+`pspdfkit_license_key` entry in that file.

android/Omnivore/secrets.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="pspdfkit_license_key">unset</string>
+</resources>