Change Readme file

- I added the info about the FFOS simulator, so that people can test the app even without owning a device.
- I added the info about translation, so that anyone who want to l10n the app can do that without forking and change by hand every file
- I changed the section title from "donate" to "contribute" because "donate" is used to give money but the most part is about contributing translating or coding. If you want to add info about Bitcoin, *coin, Paypal, ... we can create the section "Donate" just for it