ignore all keystore files

Summary:
Ignore all keystore files as we do not want users to accidentally check in their keystore file.
Closes facebook#10214

Differential Revision: D4118169

Pulled By: bestander

fbshipit-source-id: a81fc99afd1715d52dfdce8c47dfc10808470008

Author: bbodenmiller

local-cli/generator/templates/_gitignore

@@ -38,4 +38,4 @@ npm-debug.log
 buck-out/
 \.buckd/
 android/app/libs
-android/keystores/debug.keystore
+*.keystore