Added instruction to get public modules from PSGallery

[thomasrayner]

PR Summary

Fixes #1326 . An error occurs while building PSScriptAnalyzer if one is missing dependencies and has a private repository added.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
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[thomasrayner]

Since I didn't modify global.json, I'm not sure the build failure is a result of this PR.

[bergmeister]

Thanks :-)
Can't wait to see what other changes you are going to make now that you are building PSSA :-)
Hmm, the build failure on Ubuntu is odd. It's not your fault but we actually fixed a very similar problem with the Ubuntu build just this week and it's broken again :-( (the breaking changes are due to updates of the image and AppVeyor being sensitive to exit codes that the .Net teams has changed over time, we've got a PR to migrate to Azure Pipelines but ironically there are a few tests that somehow do not pass in Azure pipelines. at the moment...)
I can confirm that the current build got broken again as I just kicked off a new build on master, sorry for that

[thomasrayner]

👋 @bergmeister! I'm having too much fun messing with AST-related stuff to not try and get some of it into PSSA.

And no problem regarding the failed build stuff. Let me know if there's anything you want me to change, or commits that I should pull into my branch.