Do not try to sync project-specific files

[barmac]

Is your feature request related to a problem? Please describe.

Sync script tries to merge i.a. documentation and CI-related files. That adds unnecessary work.

Unmerged paths:
  (use "git restore --staged <file>..." to unstage)
  (use "git add/rm <file>..." as appropriate to mark resolution)
    both modified:   .travis.yml
    both modified:   CHANGELOG.md
    both modified:   README.md
    both modified:   THIRD_PARTY_NOTICES
(...)

Describe the solution you'd like

Let's exclude all md files as well as configuration from merge action.

Describe alternatives you've considered

Leave it as it is.