Document why NodeJS is needed.

[Crixu]

Right now the test suite requires NodeJS to prepare the tests and builds from a grunt file.
We should document what is actually done there as the grunt file includes more steps that might not be needed to actually run the hosting test suite.

Describe the solution you'd like
A part in the readme or the hosting handbook which describes and explains the need for NodeJS. This will make sure that hosts trying to run the hosting tests can understand the requirements.

Describe alternatives you've considered
Find a way to remove the NodeJS requirements and run the required build steps for the phpunit tests from php.