refactor: update readme.md (backport #480)

[mergify]

• Rename Frappe Health to Marley Health in readme.md
• update key-feature image

  ---

  This is an automatic backport of pull request refactor: update readme.md #480 done by Mergify.

[mergify]

Cherry-pick of b789858 has failed:

On branch mergify/bp/version-14-hotfix/pr-480
Your branch is ahead of 'origin/version-14-hotfix' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit b789858.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   README.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally