add dev helper and docs

dev/dev-env/dev-shell.nix

@@ -0,0 +1,16 @@
+pkgs:
+
+pkgs.mkShell {
+  shellHook = ''
+    # A known rev from the master branch to test whether `nix develop`
+    # is called inside the nix-bitcoin repo
+    rev=5cafafd02777919c10e559b5686237fdefe920c2
+    if git cat-file -e $rev &>/dev/null; then
+      root=$(git rev-parse --show-toplevel)
+      export PATH=$root/test:$root/helper:$PATH
+    else
+      echo 'Error: `nix develop` must be called inside the nix-bitcoin repo.'
+      exit 1
+    fi
+  '';
+}