fast clone instructions (#29455)

* fast clone instructions

* Support default ubuntu 20.04 git

---------

Co-authored-by: Harald Schäfer <harald.the.engineer@gmail.com>
old-commit-hash: 012060ba32

tools/README.md

@@ -9,10 +9,9 @@ openpilot is developed and tested on **Ubuntu 20.04**, which is the primary deve
 First, clone openpilot:
 ``` bash
 cd ~
-git clone https://github.com/commaai/openpilot.git
+git clone --filter=blob:none --recurse-submodules https://github.com/commaai/openpilot.git
 
 cd openpilot
-git submodule update --init
 ```
 
 Then, run the setup script: