Android: Updated name of README file.

android-project/AndroidManifest.xml

@@ -24,7 +24,7 @@
          then replace "SDLActivity" with the name of your class (e.g. "MyGame")
          in the XML below.
 
-         An example Java class can be found in README-android.txt
+         An example Java class can be found in README-android.md
     -->
     <application android:label="@string/app_name"
                  android:icon="@drawable/ic_launcher"

include/SDL_main.h

@@ -63,7 +63,7 @@
 /* On Android SDL provides a Java class in SDLActivity.java that is the
    main activity entry point.
 
-   See README-android.txt for more details on extending that class.
+   See README-android.md for more details on extending that class.
  */
 #define SDL_MAIN_NEEDED