Missing `git fetch llv8` in instructions

[Chaz6]

In the instructions, the following:

Fetch all the code:

fetch v8
cd v8
git remote add llv8 https://github.com/ispras/llv8.git
git checkout -b llv8 llv8/llv8
gclient sync

Should be replaced with:-

Fetch all the code:

fetch v8
cd v8
git remote add llv8 https://github.com/ispras/llv8.git
git fetch llv8
git checkout -b llv8 llv8/llv8
gclient sync

Otherwise you get an error when running git checkout.

[ivladak]

Thank you, Chaz. Updated readme and the wiki.