Add a "Help" or README in the ISO image

From the UX session :

Quite often new user do not succeed booting Tails, and go from their normal system to the Tails usb stick looking for something.

We could add an “Help” file (html, rtf… ?) and reduce if it’s possible the number of other file at / to make it the most visible.

There will be this kind of insctructions :

- Maybe you wanted to boot Tails

- You failed

- You need to find the BIOS key you have to press on

- This are the main BIOS key

- You can use a search engine with the name of your computer and BIOS to find the good keys

- You can find the name of your computer often near your screen, or at the back of your laptop

- In this page (look if there is this kind of page on the internet) you will find other insctructions to find the name of your computer
- If you need help you can try to send us an email or here (we’ll have to do this list) you will find people in your city or country that could help you

Parent Task: #14548

Original created by @tchou on 7495 (Redmine)