docs(install): Easier way to install using omz

[gavindsouza]

Changes

• Use omz command to enable plugin
• Updated git command to do a shallow clone to save space

[phy1729]

The changes to step 2 seem fine. For the changes to step 1, shallow clones aren't preferred cf. #855.

[gavindsouza]

shallow clones aren't preferred cf. #855.

Thanks for this! Updated the PR.