Installation and usage example does not work #73

Hi!

cd /ANY/PATH
git clone https://github.com/jsha/minica.git
go build
## or
# go install

The code above from the README.md causes following error message:

go: go.mod file not found in current directory or any parent directory; see 'go help modules'

Reason for that is, that the git command will create a minica directory. So after the git command a cd minica is required.

If you instead want the contents of the repository to be inserted into /ANY/PATH, the git command needs to be extended by a dot:

git clone https://github.com/jsha/minica.git .

Also the usage example should call minica with a leading ./ since build does not install the built package, but only create the compiled minica in the current directory:

./minica --domains foo.com