src: the README is added as licenseFile

[helio-frota]

This small change is only to have the README value added to the licenseFile property.

sample output:

 { 'sample_dependency@1.0.0': 
       { licenses: 'MIT*',
         licenseFile: '/full path here/sample_dependency/README' } }

In some cases we need to have the README associated to licenseFile

[davglass]

Thanks! Published in license-checker@12.0.0