Add Proguard setup section on README

Add Proguard rules for Android projects so it can be referenced right away.

README.md

@@ -136,6 +136,29 @@ try {
     //don't trust the JWT!
 }
 ```
+## Android Proguard Setup
+
+You can use the following rules on your Android projects (see [Proguard Exclusions](https://github.com/jwtk/jjwt/wiki/ProGaurd-exclusions)): 
+
+```
+-keepnames class com.fasterxml.jackson.databind.** { *; }
+-dontwarn com.fasterxml.jackson.databind.*
+-keepattributes InnerClasses
+
+-keep class org.bouncycastle.** { *; }
+-keepnames class org.bouncycastle.** { *; }
+-dontwarn org.bouncycastle.**
+
+-keep class io.jsonwebtoken.** { *; }
+-keepnames class io.jsonwebtoken.* { *; }
+-keepnames interface io.jsonwebtoken.* { *; }
+
+-dontwarn javax.xml.bind.DatatypeConverter
+-dontwarn io.jsonwebtoken.impl.Base64Codec
+
+-keepnames class com.fasterxml.jackson.** { *; }
+-keepnames interface com.fasterxml.jackson.** { *; }
+```
 
 ## Supported Features