README.md: Point to developer guide for git details

README.md

@@ -196,7 +196,7 @@ in the Installation Guide.
  - [Cygwin] Avoid building in home directories of Windows domain
  users or in paths with capital letters.
 
-2. Download/unpack the sources.
+2. Download/unpack or clone the sources.
 
  - Go to https://www.sagemath.org/download-source.html, select a mirror,
  and download the file :file:`sage-x.y.tar.gz`.
@@ -214,12 +214,16 @@ in the Installation Guide.
 
   $ cd sage-x.y/ # adapt x.y
 
- - [Git] Alternatively, clone the Sage git repository:
+ - [Git] Alternatively, and required for Sage development, clone the Sage
+ git repository:
 
   $ ORIG=https://github.com/sagemath/sage.git
   $ git clone -c core.symlinks=true --branch develop --tags $ORIG
 
- This will create the directory `sage`.
+ This will create the directory `sage`. (See the section
+ [Setting up git](https://doc.sagemath.org/html/en/developer/git_setup.html)
+ and the following sections in the Sage Developer's Guide
+ for more information.)
 
  Change into it and pick the branch you need, typically
  the latest development branch: