chore: add yarn logs to gitignore (#13884)

Adds the yarn-related logs to a gitignore so people don't try to commit them accidentally. See #13814.

Author: crisbeto

.gitignore

@@ -36,6 +36,8 @@ node_modules
 /coverage/*
 /libpeerconnection.log
 npm-debug.log
+yarn-debug.log
+yarn-error.log
 testem.log
 /.chrome
 /.git