[FEATURE REQ] Clean up the "Repository Contents" section of the README

[minas1]

The Repository Contents sections of the README is confusing. To an android developer like myself, it's not clear which sample I should consult in order to use the library in my app.

There's:

1. notification-hubs-sample-app-java
2. notification-hubs-sample-app-legacy
3. FCMTutorialApp

For a new user of the library, the relevant one seems to be the last one (FCMTutorialApp). Whichever it is, it should be the first one in Repository Contents sections.

The rest should be below that one, preferably in a section with a lower heading.

---

Example:

Repository Contents

FCMTutorialApp
[...]

Legacy

notification-hubs-sample-app-java
[...]

notification-hubs-sample-app-legacy
[...]

Other

com.microsoft.windowsazure.messaging
[...]

notification-hubs-sdk-e2etestapp
[...]