Add note to readme about snapshot builds

[filipesilva]

We publish snapshot builds of all devkit packages, but do not mention it anywhere. We should mention this in the readme so users can take advantage of the snapshots.

[maxtacco]

Hi @filipesilva, I'm trying to install the latest build of angular-devkit/build-ng-packagr that has custom tsconfig support. Could you please point me into the right direction?

[filipesilva]

Hi @maxtacco, you can use https://github.com/angular/angular-devkit-build-ng-packagr-builds at master.

[maxtacco]

Thank you very much! Really appreciate your quick response.