Document how to recover from corrupted nimble cache
I somehow got a situation where my project wouldn't build and gave me the following error:

```shell
$ nimble build --verbose                           
     Error: Could not read package info file in /Users/hitesh/dev/nim-d3/example
s/examples.nimble;                                                              
        ...   Reading as ini file failed with:                                  
        ...     Invalid section: .                                              
        ...   Evaluating as NimScript file failed with:                         
        ...                                                                     
        ... printPkgInfo() failed.
```

After being very confused with this error I stumbled upon the `--debug` option.

```shell
$ nimble build --debug                             
     Debug: Executing nim e --hints:off --verbosity:0 -p:/var/folders/f4/gzyr8mc
n5yv0jvdv5l47zygr0000gn/T/nimblecache /Users/hitesh/dev/nim-d3/examples/examples
_61054.nims /var/folders/f4/gzyr8mcn5yv0jvdv5l47zygr0000gn/T/nimble_61054.out pr
intPkgInfo                                                                      
     Error: Could not read package info file in /Users/hitesh/dev/nim-d3/example
s/examples.nimble;                                                              
        ...   Reading as ini file failed with:                                  
        ...     Invalid section: .                                              
        ...   Evaluating as NimScript file failed with:                         
        ...                                                                     
        ... printPkgInfo() failed.
```

Deleting the following resolved the issue and my builds succeeded again.

```text
/var/folders/f4/gzyr8mcn5yv0jvdv5l47zygr0000gn/T/nimblecache/*
/var/folders/f4/gzyr8mcn5yv0jvdv5l47zygr0000gn/T/nimble_61054.out
```

This kind of build failure is not easy to track down unless you know these steps.  It would be nice to have nimble be more fault tolerant and not let the user get into this situation in the first place.  But at the very least, this should be documented in the troubleshooting section.
