Add development information to readme

README.md

@@ -1,6 +1,7 @@
 # Circle Stopwatch
 
-__IMPORTANT: Repo and code will improve over time. Everything, including the repo itself, should be treated as work in progress right now.__
+__IMPORTANT: Repo and code will improve over time. Everything, including the__
+__repo itself, should be treated as work in progress right now.__
 
 ----
 
@@ -9,4 +10,21 @@ __IMPORTANT: Repo and code will improve over time. Everything, including the rep
 This repository contains the code for "Circle Stopwatch", originally released
 as a Silverlight app for Windows Phone 8.1.
 
-The code will improve over time and more features will be added. This repository will move to GitHub eventually.
+The code will improve over time and more features will be added. This
+repository will move to GitHub eventually.
+
+## Development
+
+### Requirements
+
+- Visual Studio 2015 (Community Edition works fine)
+- Windows Phone 8.1 SDK
+- Windows Phone 8.1 Update 1 (GDR1) Emulators (optional) or an actual phone
+
+If you want to use a real phone for development, make sure it's either
+registered as a developer device (which you cannot do anymore) or its
+bootloader is unlocked and XAP sideloading was activated using Xapload
+Unlocker.
+
+To deploy XAPs, use the _Windows Phone Application Deployment_ Tool bundled
+with the Windows Phone 8.1 SDK.