Add Linux AppImage to nightly build download links #885
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
Motivation
Linux x86-64 builds of the Arduino IDE are now available in AppImage format in additional to the previous ZIP format (#846).
Since only the AppImage format IDE supports auto-updates (the IDE installed from the ZIP will notify of available updates, but can't auto-update), this will be the preferred format and so good beta testing coverage of it is especially important.
Change description
Add Linux x64-64 AppImage option to the "Nightly builds" download link table in the readme.
Other information
Reviewer checklist