docs(README.md): add value for dest to conventionalChangelog settings for Gruntfile.js

[mikeebowen]

the module won't write the file unless you include a "dest" value in the "release" object in the conventionalChangelog settings in the Gruntfile.js. without it, the file is never created

[coveralls]

Coverage remained the same at 94.737% when pulling 8b7816b on mrbgit:master into e15a5a2 on btford:master.

[stevemao]

dest should be the same as src if ignored

[mikeebowen]

dest should be the same as src if ignored, but that's not the current behavior. Currently, if it's not added to the release object, it doesn't create the CHANGELOG.md file file.

[stevemao]

This should be treated as a bug I think.