more stars for k-paxian dart-package-publisher

[lcuis]

Hello @k-paxian ,

I find your dart-package-publisher action really useful and I believe it deserves more stars. This is why I am proposing this PR with improvements that may have helped me choose this action quicker from the marketplace:

• "Flutter" in the action name.
• Repeat description in the README because otherwise it is not visible in the action page and the page starts with "The Challenge" with no introduction.
• Mentioned more often the fact that this action also applies for Flutter.
• Added the diagram below explaining "The Challenge" to the README. I prepared it with draw.io. I placed the source and the png in the new doc/images folder.
• Added the possibility to use only one parameter for the whole credential json (credentialJson) from the secrets instead of two separate tokens. This is a solution I have seen in another action.

:warning: Warning, I was not able to test the credentialJson as this would have probably required me to publish the action to the marketplace, which is not my intention.

[k-paxian]

@lcuis Thank you so much for your contribution!

[lcuis]

Hello @k-paxian , Thank you very much for your reply, the merge and the package deployment! I was able to test a version match with the credentialJson set without any token and it seemed to work as expected:

...
Local version: [1.0.4]
Remote version: [1.0.4]
Remote & Local versions are equal, skip publishing.