docs: Add Nix API reference docs

docs/nix-reference.md

@@ -0,0 +1,20 @@
+# Gomod2nix Nix API
+
+## Public functions
+
+### buildGoApplication
+Arguments:
+- **modules** Path to gomod2nix.toml (_default: `pwd + "/gomod2nix.toml"`).
+- **src** Path to sources (_default: `pwd`).
+- **pwd** Path to working directory (_default: `null`).
+- **go** The Go compiler to use (can be omitted).
+- **subPackages** Only build these specific sub packages.
+- **allowGoReference** Allow references to the Go compiler in the output closure (_default: `false`).
+
+All other arguments are passed verbatim to `stdenv.mkDerivation`.
+
+### mkGoEnv
+Arguments:
+- **pwd** Path to working directory.
+
+All other arguments are passed verbatim to `stdenv.mkDerivation`.