docs: add browser environment to common gotchas

Author: eddyerburgh

docs/en/common-gotchas.md

@@ -1,5 +1,11 @@
 # Common gotchas
 
+## browser environment
+
+Vue Test Utils must be run in a browser environment. This means you can run it in a browser (not recommended), or you can run it in node using a virtual browser, like [JSDOM](https://github.com/tmpvar/jsdom).
+
+[Jest](https://facebook.github.io/jest/docs/en/cli.html) sets up JSDOM automatically, so it is the easiest way for you to run Vue Test Utils in node.
+
 ## createLocalVue
 
 `createLocalVue` returns an extended Vue class so you can add mixins, directives, components and plugins without affecting the global Vue class.