Document how to override packages in nix

[srid]

@asheshambasta asked:

with reflex platform, how does one add an overlay to pkgs being used by rp.project? I've so far found no documentation on this. Say a package is marked as broken and I'd want to add an overlay that fixes the breakage, how does one achieve that?

[srid]

Short answer: https://github.com/obsidiansystems/obelisk/blob/master/FAQ.md#how-do-i-add-or-override-haskell-dependencies-in-the-package-set

We can perhaps link to this from the README.

[asheshambasta]

@srid Another good resource is the nix expression itself: https://github.com/reflex-frp/reflex-platform/blob/develop/project/default.nix.
Maybe adding this to the readme for people who'd like to know about all knobs they can tune (at least that's how I got to using overrides.)

And since we're dealing with overrides, the usual caveats with composing overrides apply.