Mention Eclipse dropins folder

Fixes #134, Fixes #135

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171386443

Author: sormuras

README.md

@@ -44,7 +44,9 @@ Settings...".
 
 A [google-java-format Eclipse
 plugin](https://github.com/google/google-java-format/releases/download/google-java-format-1.3/google-java-format-eclipse-plugin-1.3.0.jar)
-can be downloaded from the releases page.
+can be downloaded from the releases page. Drop it into the Eclipse [dropins
+folder](http://help.eclipse.org/neon/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Freference%2Fmisc%2Fp2_dropins_format.html)
+to activate the plugin.
 
 The plugin adds a `google-java-format` formatter implementation that can be
 configured in `Window > Preferences > Java > Code Style > Formatter > Formatter