Missing build and test instructions

[pires]

As per https://github.com/sourcegraph/checkup/blob/master/CONTRIBUTING.md#getting-started, there should be build and test instructions.

[bishgupp]

Re: Build and test instructions... I get an '...checkup.json..' file missing error message when I run the 'checkup.exe' on a 64bit Windows PC

[rileytg]

i use

go run cmd/checkup/main.go 
go test .

[JonathanMH]

I've experienced, that if you don't clone the repo into the expected path, it will fail too:

cmd/checkup/main.go:3:8: cannot find package "github.com/sourcegraph/checkup/cmd" in any of:
    /usr/local/go/src/github.com/sourcegraph/checkup/cmd (from $GOROOT)
    /home/user/projects/go/src/github.com/sourcegraph/checkup/cmd (from $GOPATH)

Maybe that's a general go thing, I have no quite understood all the dependency management fully.

[mholt]

I hope these clarifications to the contributing guide help a bit.