[FirebasAuth] added that Errors are thrown via a PlatformException - #146

Description

When I started with FirebaseAuth I was puzzled how I access possible error code because exceptions get easily swallowed in an async function.
I added a paragraph to the readme and to the code comments to make it easier for people just starting with Firebase and Flutter

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• [ x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [ x] If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• [ x] I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• [ x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• [ x] I signed the [CLA].
• [ x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• [x ] No, this is not a breaking change.