Add build instructions for contributors

Could we please add a `BUILD.md` or `CONTRIBUTING.md` that explains some of the ways to build this project?

```
hackage-server> cabal new-build all
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: hackage-server-0.5.1 (user goal)
[__1] next goal: rss (dependency of hackage-server)
[__1] rejecting: rss-3000.2.0.6 (constraint from project config TODO requires
<3000.2.0.6 || >3000.2.0.6)
[__1] trying: rss-3000.2.0.5
[__2] trying: HaXml-1.25.4 (dependency of rss)
[__3] next goal: base (dependency of hackage-server)
[__3] rejecting: base-4.11.1.0/installed-4.1... (conflict: HaXml
+/-bytestringinbase +/-splitbase => base<4.11)
[__3] rejecting: base-4.11.1.0, base-4.11.0.0, base-4.10.1.0, base-4.10.0.0,
base-4.9.1.0, base-4.9.0.0, base-4.8.2.0, base-4.8.1.0, base-4.8.0.0,
base-4.7.0.2, base-4.7.0.1, base-4.7.0.0, base-4.6.0.1, base-4.6.0.0,
base-4.5.1.0, base-4.5.0.0, base-4.4.1.0, base-4.4.0.0, base-4.3.1.0,
base-4.3.0.0, base-4.2.0.2, base-4.2.0.1, base-4.2.0.0, base-4.1.0.0,
base-4.0.0.0, base-3.0.3.2, base-3.0.3.1 (constraint from non-upgradeable
package requires installed instance)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: HaXml, rss, base, hackage-server
```

```
hackage-server> cabal --version
cabal-install version 2.2.0.0
compiled using version 2.2.0.0 of the Cabal library
hackage-server> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.4.2
```

For instance for a `cabal` build we might say to use `cabal new-build` with `project.local` making sure that there is a matching version of `ghc` available.