Add information to the README about homebrew on linux

[chmouel]

Changes

Since we are compatible with Homebrew on Linux :

let's add it to the list

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

See the contribution guide
for more details.

Release Notes

tektoncd-cli can be install with [Homebrew on Linux](https://docs.brew.sh/Homebrew-on-Linux).

[pradeepitm12]

/lgtm

[piyush-garg]

/lgtm

[tekton-robot]

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