mention build dependencies in the README

[danieleades]

Diesel has requirements on system packages, depending on which features are installed. These are the drivers for the different databases.

The fact that these dependencies must be installed (or you get linker errors during build) should be mentioned

for example, libpq (package libpq-dev) is a required system dependency when using the diesel/postgres feature, which is enabled by default

[Hirevo]

Yes, we're indeed have system dependencies that could be mentioned.
I'll go compile the project with a bunch of different features to see which system dependencies end up being required.