update build instructions

Signed-off-by: Grant Limberg <glimberg@gmail.com>

java/README.md

@@ -1,4 +1,17 @@
 ZeroTier One SDK - Android JNI Wrapper
 =====
 
-To Build:  <path to Android-ndk>/ndk-build ZT1=<path to ZeroTierOne root>
+
+Building
+-----
+
+Reqires:
+
+* JDK
+* ANT
+* Android NDK
+
+Required Environment Variables:
+
+* NDK\_BUILD\_LOC - Path do the ndk-build script in the Android NDK
+* ANDROID\_PLATFORM - path to the directory android.jar lives (on Windows: C:\Users\<username>\AppData\Local\Android\sdk\platforms\android-21)