Update gitignore according to yarn docs

Author: ejscribner

.gitignore

@@ -40,6 +40,13 @@ build/Release
 # Dependency directories
 node_modules/
 jspm_packages/
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions
 
 # TypeScript v1 declaration files
 typings/