Add android_alarm_manager package to README

[alibitek]

Travis build fails due to: flutter/flutter#13813 , fixed in flutter/flutter#13814

[mit-mit]

@alibitek this looks like just a change to the readme (as the title says), can you clarify the travis part in our description?

[alibitek]

@mit-mit The change in this PR doesn't have anything to do with the build failing.

I mentioned that because the build was failing in macOS for the entire flutter plugins repository, due to the mentioned CocoaPods issue affecting the master branch of the flutter repository. (the place from where the Podfile for the android_alarm_package project is generated)

The issue was fixed in the flutter plugins repository for the android_alarm_package in a recent PR, by creating a Podfile for this package, thus bypassing the generation.

I've triggered another Travis build for the change in this PR by:

git rebase upstream/master
git checkout android_alarm_manager_readme
git commit --amend
git push --force-with-lease origin android_alarm_manager_readme

However, the build still has failing jobs outside the scope of the change and there is no way for me to tell Travis to rebuild, unless I run the above commands again or close and reopen the pull request.
e.g. https://travis-ci.org/flutter/plugins/jobs/324550433#L642

Picked up _JAVA_OPTIONS: -Xmx2048m -Xms512m
Warning: An error occurred while preparing SDK package Android SDK Build-Tools 26.0.3: Connection timed out (Connection timed out).
The command "echo y | sdkmanager "build-tools;26.0.3" >/dev/null" failed and exited with 1 during .

Encountered this problem several times with my other pull requests, where Travis build jobs fail due issues (timeouts, server overload, etc.) not related to the changes made by the pull request.
I think there should be a way from the GitHub or Travis Web UI where people submitting pull requests can tell Travis to trigger a restart even when they do not have write access to the repository/collaborator status.

For more on this see: travis-ci/beta-features#27 (comment)

Back to this pull request: After yet another manual rerun all the build jobs are successful :-)

[mit-mit]

Got it, thanks for clarifying! Yes, it's sad that only admins can re-run.

Readme change LGTM