Exclude markdown files from 'Pester' folder

[daxian-dbw]

PR Summary

Exclude markdown files from powershell.AppDir folder which is temporary for AppImage creation.
From https://travis-ci.org/PowerShell/PowerShell/jobs/312145407

The command "pwsh -File tools/travis.ps1" exited with 0.
1.82s$ if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then mdspell '**/*.md' '!**/publish/**/*.md' --ignore-numbers --ignore-acronyms --report; fi
    powershell/powershell.AppDir/usr/bin/Modules/Pester/CHANGELOG.md
        2 | ed versions of that command. [GH-362] 

PR Checklist

Note: Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• [NA] Update the changelog
• [NA] User facing Documentation needed
  • [NA] Issue filed - Issue link:
• Change is not breaking
• [NA] Make sure you've added a new test, if existing tests do not effectively test the code changed
  • [NA] Add [feature] if the change is significant or affectes feature tests

[TravisEz13]

should be run with [package]

[TravisEz13]

should be verified with a [package] commit

[JamesWTruher]

LGTM