Document path.existsSync()

Closes GH-677.

doc/api/path.markdown

@@ -112,3 +112,8 @@ with either true or false. Example:
     path.exists('/etc/passwd', function (exists) {
       util.debug(exists ? "it's there" : "no passwd!");
     });
+
+
+### path.existsSync(p)
+
+Synchronous version of `path.exists`.