github-setup: Clarify requirements and remove mentions of the VM
github-setup: Clarify requirements and remove mentions of the VM

- New starters shouldn't concern themselves with the VM straight away.
- Mention that it's OK to use your existing (personal) GitHub account if
  you have one, people don't need to make a GDS-specific one.
- We don't all use Macs. :-)

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs