improve README (#4023)

* improve README

include instructions on how to run the tests

* fix language

Co-authored-by: Stanislaw Findeisen <sfindeisen@users.noreply.github.com>

Author: sfindeisen

README.md

@@ -64,6 +64,8 @@ https://github.com/input-output-hk/jormungandr/releases/latest
 git clone https://github.com/input-output-hk/jormungandr
 cd jormungandr
 git checkout tags/<latest release tag> #replace this with something like v1.2.3
+cargo build # skip this if you do not want to run the tests
+cargo test  # skip this if you do not want to run the tests
 cargo install --locked --path jormungandr # --features systemd # (on linux with systemd)
 cargo install --locked --path jcli
 ```