[Proposal] SCO-0001: Generic file providers (#62)

### Motivation

Check out the proposal text:
https://github.com/czechboy0/swift-configuration/blob/hd-sco-0001/Sources/Configuration/Documentation.docc/Proposals/SCO-0001.md

### Modifications

Added the proposal, the draft implementation can be found in
#60.

### Result

N/A

### Test Plan

N/A

Author: czechboy0

Sources/Configuration/Documentation.docc/Proposals/Proposals.md

@@ -36,3 +36,4 @@ If you have any questions, ask in an issue on GitHub.
 ## Topics
 
 - <doc:SCO-NNNN>
+- <doc:SCO-0001>