[docs] Improved docs for installing upcoming version #304
[docs] Improved docs for installing upcoming version #304

Issue:
Instructions for installing dependencies were missing

Changes:
- Use ansible-galaxy to install ansible-openwisp2 role. This
  automatically installs dependencies.
- Set recommended ansible version to 2.10

Closes #304

nemesisdesign/ansible-openwisp2,openwisp/ansible-openwisp2