Added dependencies and steps to build in README

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Update the README a bit

There where some inconsistencies as build.sh evolved after Venkatesh
wrote his patch.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Fixup and simplify libgit2 building

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Better way of downloading from git

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Update libdivecomputer and subsurface

Current subsurface needs two patches for NO_MARBLE-building, but those
are pending for merge right now.

Signed-off-by: Anton Lundin <glance@acc.umu.se>