Installation instructions

[kroggen]

Hi!

The installation instructions are not clear.

They appear not be in the order they should be executed. Example: the requirements should come first.

And it is not clear whether the installation of secp256k1 and the USB tricks on Linux should be executed on the virtualenv or outside of it.

Also, how to add these rules if rules.d is a folder?

[kroggen]

Maybe instead of those permissions currently on the README we could use:

wget -q -O - https://raw.githubusercontent.com/LedgerHQ/udev-rules/master/add_udev_rules.sh | sudo bash