docs: [#56] update README for integration tests

Author: josecelano

tests/README.md

@@ -1,10 +1,21 @@
-### Running Tests
-Torrust requires Docker to run different database systems for testing. [install docker here](https://docs.docker.com/engine/).
+# Running Tests
+
+Torrust requires Docker to run different database systems for testing. [Install docker here](https://docs.docker.com/engine/).
 
 Start the databases with `docker-compose` before running tests:
 
-    $ docker-compose up
+```s
+docker-compose -f tests/docker-compose.yml up
+```
 
 Run all tests using:
 
-    $ cargo test
+```s
+cargo test
+```
+
+Connect to the DB using MySQL client:
+
+```s
+mysql -h127.0.0.1 -uroot -ppassword torrust-index_test
+```