* `mdocspec` package parses ocpath.proto and ocrpcs.proto from yaml
  according to the format shown below by mdocspec.Parse's documentation.
* `tools/validate_readme_spec/validate_readme_spec.go` script is added
  that can parse and then validate OC Paths and RPCs according to
  existing repo states in github.com/openconfig.

```go
// Parse extracts sorted OpenConfig Path and RPC Coverage from an FNT
README.
//
// The first yaml code block after a heading line named exactly as
// "OpenConfig Path and RPC Coverage" will be used. Any other code
blocks are
// ignored.
//
// Expected markdown format:
//
//      ## OpenConfig Path and RPC Coverage
//
//      ```yaml
//      paths:
//        /interfaces/interface/config/description:
//        /interfaces/interface/config/enabled:
//        /components/component/state/name:
//          platform_type: "CHASSIS"
//
//      rpcs:
//        gnmi:
//          gNMI.Set:
//            union_replace: true
//          gNMI.Subscribe:
//            on_change: true
//      ```
//
// Note: For `rpcs`, only the RPC name and methods are validated. Any
// attributes defined below RPC methods (e.g. union_replace) are not validated.
```

Co-authored-by: Darren Loher <dloher@google.com>

… mdocspec

* Skip repo-cloning during validation if repo already cloned.
* Validate that at least one RPC method is specified for any coverage spec.
* Add additional validation for script in bash file.
* Style Improvements.