Semver-style Changelogs for Resource Provider Swagger Definitions

This was previously requested in #1770 - but the issue was closed without being resolved - as such would it be possible to start putting together Changelog's for each Resource Provider? These could then be collated into the Azure SDK's at release time. As a consumer - I'd started looking for this in both the Resource Provider folder or in the folder containing the API versions (but didn't see it in either).

More details can be found in #1770 - but generally speaking we're looking for something like this:

```
# Compute API

## Version 2019-01-01:

NEW FEATURES:

* Virtual Machines: support for toggling on/off Disk Encryption
* Disks: Support for super-fast disks

CHANGES:

* Virtual Machines: the Delete API now requires that Virtual Machines must be shut down prior to the Delete API being called

DEPRECATIONS:

* Virtual Machines: Unmanaged Disks are now deprecated
```

Thanks!

Review the requirements and prior discussion in issue #1770, then inspect the Resource Provider folders and API-version folders where the requester expected changelogs. Define how per-provider, semver-style entries would be maintained and collated into Azure SDK releases; done means the location, format, and release process are specified and implemented across the relevant definitions.

Written by the indexing model from the issue text.