Add build instructions

[FennyFatal]

README.md could use some build instructions.

Perhaps something like:

git clone https://github.com/boppreh/steamgrid
cd steamgrid
echo "module github.com/boppreh/steamgrid" > go.mod
go mod tidy
go build

But it might be better to include a go.mod file in the repo rather than create it at build time.