Add HACKING md file

HACKING.md

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+F-Droid Client Hacking Doc
+==========================
+
+Style
+-----
+
+We follow the [Google Java style](https://google-styleguide.googlecode.com/svn/trunk/javaguide.html).
+Some of its points are:
+
+ * Four space indentation
+ * UTF-8 source files
+ * Exactly one top-level class per file
+ * No wildcard imports
+ * Braces are always used and follow K&R
+ * Commented fallthroughs
+
+We didn't follow any style before but generally it was more or less like
+Google's. As we make the switch, we encourage devs to follow this new format.
+
+Building
+--------
+
+You have three options:
+
+ * Build with gradle
+ * Build with gradle from source
+ * Build with ant from source
+
+Any will work, but if you use gradle, remember that you can use --daemon to
+not have to watch gradle load every time.