Readme.md : not all the installations steps are clear enough

[lycenok]

For the step 2:
you should add:
"The database $DB_NAME should be already created"

For the step 3 you should mention where phpunit must run from:

3. Go to your magento root directory and run unit tests for the first time to install test database by running phpunit. It should take about 3 minutes.
   $ phpunit

Spent some time to figure it out.

[lycenok]

Also I needed to download vfsStream to lib directory manually.