Add Realm.schemaVersion API doc

This may need to be adjusted depending on the decisions surrounding this API.

docs/realm.js

@@ -103,6 +103,14 @@ class Realm {
     write(callback) {}
 }
 
+/**
+ * Get the current schema version of the Realm at the given path.
+ * @param {string} path - The path to the file where the
+ *   Realm database is stored.
+ * @returns {number} version of the schema, or `-1` if no Realm exists at `path`.
+ */
+Realm.schemaVersion = function(path) {};
+
 /**
  * The default path where to create and access the Realm file.
  * @type {string}