Clean up installation instructions

[johntoomey]

Specifically - add a note about older versions of ONIE causing an issue

Signed-off-by: John Toomey john@toganlabs.com

repo: mion-docs

• Issue: https://github.com/NetworkGradeLinux/meta-mion/issues/63

• Style-guide check [y/n]:

• Rendered page viewed with: mkdocs/mike, grip...

• If instructional,

  • Affected hardware: all -or- specific switches
  • Verified: does it take you from a -> b without issue

• Description: brief description of issue or enhancement

[kprosise]

Changes don't adhere to the mion style guide, so will not be accepting this pull request. However, the page does need to be updated. @johntoomey, you can either resubmit or assign the issue to me and I'll make the necessary changes and include them with #241