Update install from source instructions: add --recurse-submodules (#4081)

Update the installation from source instructions to also copy typeshed 
(see #3867). Also minor formatting improvements.

Author: ilevkivskyi

docs/source/getting_started.rst

@@ -6,7 +6,8 @@ Getting started
 Installation
 ************
 
-Mypy requires Python 3.3 or later.  Once you've `installed Python 3 <https://www.python.org/downloads/>`_, you can install mypy with:
+Mypy requires Python 3.3 or later.  Once you've `installed Python 3 <https://www.python.org/downloads/>`_,
+you can install mypy with:
 
 .. code-block:: text
 
@@ -15,10 +16,11 @@ Mypy requires Python 3.3 or later.  Once you've `installed Python 3 <https://www
 Installing from source
 **********************
 
-To install mypy from source, clone the github repository and then run pip install locally:
+To install mypy from source, clone the github repository and then run
+``pip install`` locally:
 
 .. code-block:: text
 
-    $ git clone https://github.com/python/mypy.git
+    $ git clone --recurse-submodules https://github.com/python/mypy.git
     $ cd mypy
     $ sudo python3 -m pip install --upgrade .