Document best practices

Here is a draft for what the *best practices* should contain. Follow-up of #18.

## Topics

### No console.log() on non-primitive values in tests

As @mjhea0 pointed out, the `console.*` methods will keep a reference to any object/array/function you pass them, thus artificially introducing a memory leak. See http://stackoverflow.com/a/28859493/1799408.

### Leakage tests are quite slow
... so think twice before running it together with your other tests everytime. Point out why they are quite slow (as in the FAQ/Timeouts). Ideally present alternatives.

### Make test fail first, then fix it
Similar to other kinds of test it is also best practice to first check that your leakage test actually works, by purposely introducing a leak and watching the test fail. Remove the initial leak again and make sure the test passes.

*(to be filled with additional content)*