Mention dependencies for building the debian package

Hi there,

just tried to build the debian package using "make builddeb" on Ubuntu 16.04. Just thought, it might be worth mentioning in the doku that (at least) the following packages need to be installed:

devscripts (for buildep)
musl-tools
debhelper
help2man
python-pytest
python-mock

Kind Regards,
Beet

I think this mentions the build dependencies:

dumb-init/debian/control

Lines 6 to 12 in c7c6b00

    
           Build-Depends: 
        
            debhelper (>= 9), 
        
            help2man, 
        
           ## Tests: 
        
            python, 
        
            python-pytest, 
        
            python-mock,

The usual debian way to install this is with mk-build-deps

Yup, mk-build-deps (from devscripts) is the preferred way to do this. I don't think we want to duplicate the actual build dependencies in the README, but I'll add some additional info to the README about how to build (and install the deps)!

	Build-Depends:
	debhelper (>= 9),
	help2man,
	## Tests:
	python,
	python-pytest,
	python-mock,