Add / Improve build instructions

[kennethdhau]

Sorry if this is a repeat/mentioned somewhere, but is there a makefile/cmake/scons setup if I'm compiling from source?

[tehhowch]

Install QtCreator, and possibly also the relevant Qt basics (i.e. Qt5)
On Ubuntu 16.04 I am able to build and run by installing qtcreator-dev and qt5-default (and the associated required support packages that apt-get mentions). Installing a lesser qtcreator package may be permissible.
After installing those packages, you can open QtCreator and point it at the .pro file.

(The need to install QtCreator is mentioned in the README.md file. As the project uses types fundamental to Qt, you'll need Qt to build it.)