Enhance Clients And Connectors docs

[dpordomingo]

discovered at https://github.com/src-d/empathy-sessions/issues/37#issuecomment-470046657
related to https://github.com/src-d/engine/issues/298

In Clients and Connectors section from docs, it is said that

to connect to the language parsing server (Babelfish) and analyzing the UAST, there are several language clients

I think it should be mentioned that:

• it is feasible because Engine exposes gitbase and bblfsh containers under certain ports
• gitbase is also reachable with many mysql clients

Until https://github.com/src-d/engine/issues/298 is not solved, it could be mentioned that the following workaround would run bblfsh and engine for you:

$ srcd ini
$ srcd sql "select *"

I also think it could be useful to provide some examples:

• using native clients:

    $ mysql \
      --user=root \
      --host=127.0.0.1 \
      --port=3306 \
      --execute "SHOW tables"

• and custom scripts, like for example using go-client and python commands like done by dpordomingo/empathy-sessions/4-engine/cmd.