Documentation for old IOTA-UL users explaining how to migrate to IOTA-JSON

[fgalan]

Now that IOTA-UL is officially deprecated (see telefonicaid/iotagent-ul#695) a documentation explaining how to migrate from it to IOTA-JSON would be desirable.

[dherykw]

Agree :(

[AlvaroVega]

First approach: #882