/component-selenium-grid - Updated readme file

Rewrote readme file to improve clarity and flow.
this is also a service now.
Service is created by launch, not by "Add Component", and than added as a service in env.

I would suggest:
Step 3. Create if not exists Amazon Cloud Account
Step 4. Confirm: *AWS Cloud Account*, *Workflow Service* and *Secure Vault 2.0* running for your environment ...