Document build instructions

Document the dependencies and build instructions.

Compiling

sudo apt install -y libdb-dev libgpgme-dev libbz2-dev automake
git clone https://gitlab.hax0rbana.org/public-repos/reprepro
cd reprepro/
git checkout xz-support
./autogen.sh
./configure
make

Deb package

sudo apt install -y build-essential devscripts debhelper libarchive-dev shunit2 db-util libgpgme-dev libdb-dev libbz2-dev liblzma-dev
git clone https://gitlab.hax0rbana.org/public-repos/reprepro
cd reprepro/
git checkout xz-support
debuild --no-sign --build=binary
ls ../*.deb