Add missing build instruction git am

[MarcelRobitaille]

After running ./new-driver in sigrok-util, you have to run git am <...>.patch.

I also added some commands you can copy-paste to quickly download the required source files.

[MarcelRobitaille]

Actually, I think the best way would just be to have people clone your fork.

1. git clone https://github.com/pico-coder/libsigrok.git
2. cd libsigrok
3. ./autogen.sh
4. ./configure
5. make
6. sudo make install

Or just git clone https://github.com/pico-coder/libsigrok.git and then follow the instructions in that repository's readme.