update README with build and password instructions

README.md

@@ -16,9 +16,18 @@ Requirements:
 
 To fetch and build:
 
-    go get -u layeh.com/barnard
+    git clone https://layeh.com/barnard
+    cd barnard
+    make build
 
-After running the command above, `barnard` will be compiled as `$(go env GOPATH)/bin/barnard`.
+Or install system-wide:
+
+    make install
+
+The server password can be provided via the `MUMBLE_PASSWORD` environment
+variable (preferred over the `-password` flag, which is visible in process
+listings). Use `-password-prompt` to read the password interactively from
+stdin.
 
 ## Manual