Document build

[martijnhoekstra]

It's unclear how the build and packaging functions. Particularly, versioning is done in the GitVersion build task, but dotnet build doesn't function with this build task.

I can't find out how to create a local package and how to version it.

[barrett777]

I think we've figured out most of this in the other issue: #97

Future users can click on the build status badge to see how travis ci builds the library (https://travis-ci.com/barrett777/Heroes.ReplayParser). I think that is good enough for now.