Move cabal.project to contrib/

- Update documentation for using it.

development.markdown

@@ -29,12 +29,15 @@ That will generate a `<executable-name>.prof` plain text file with profiling dat
 ## Building with cabal
 
 Unison can also be built/installed with cabal. You'll need ghc 8.6.x to
-successfully build its dependencies.
-
-* To configure the build, you can use e.g. `cabal v2-configure --with-ghc=ghc-8.6.5`
-if you have multiple versions of GHC installed
-* To build all projects use `cabal v2-build all`
-* Tests can be run with e.g. `cabal v2-run parser-typechecker:tests`
-* The executable can be installed with `cabal v2-install unison` the install
-  directory can be modified by setting `installdir: ...` or`install-dirs`
-  in `.cabal/config`
+successfully build its dependencies. The provided project file is also in
+contrib/ so you'll need to specify its location on the command line.
+
+* To configure the build, you can use e.g. `cabal v2-configure
+  --project-file=contrib/cabal.project --with-ghc=ghc-8.6.5` if you have
+  multiple versions of GHC installed
+* To build all projects use `cabal v2-build --project-file=contrib/cabal.project all`
+* Tests can be run with e.g. `cabal v2-run --project-file=contrib/cabal.project
+  parser-typechecker:tests`
+* The executable can be installed with `cabal v2-install
+  --project-file=contrib/cabal.project unison` the install directory can be
+  modified by setting `installdir: ...` in `.cabal/config`