Derive Read for Scheme using deriving in BaseUrl

[nattybear]

This PR adds the Read instance to the Scheme data type in BaseUrl by extending the deriving clause.
No other changes are made. This allows parsing Scheme values from strings using the standard Read mechanism.

I understand the drawbacks of using Read, as explained in this link. However, since the readable function in the config-ini library relies on Read, this PR became necessary.

Thank you for allowing this PR.

[tchoutri]

@nattybear Thank you! Could you please add a changelog entry (as shown in https://github.com/haskell-servant/servant/blob/master/CONTRIBUTING.md#changelog-entries)? Other than that, it's good for me. :)

[nattybear]

As requested, I’ve added a file to changelog.d. This PR system is new to me, but it looks really useful!