README.md: Update section Directory Layout

README.md

@@ -349,54 +349,72 @@ Simplified directory layout (only essential files/directories):
 SAGE_ROOT Root directory (sage-x.y.z in Sage tarball)
 ├── build
 │ └── pkgs Every package is a subdirectory here
-│ ├── 4ti2
+│ ├── 4ti2/
 │ …
-│ └── zn_poly
+│ └── zn_poly/
 ├── configure Top-level configure script
 ├── COPYING.txt Copyright information
 ├── pkgs Source trees of Python distribution packages
 │ ├── sage-conf
 │ │ ├── sage_conf.py
 │ │ └── setup.py
 │ ├── sage-docbuild
-│ │ ├── sage_docbuild
+│ │ ├── sage_docbuild/
+│ │ └── setup.py
+│ ├── sage-setup
+│ │ ├── sage_setup/
 │ │ └── setup.py
 │ ├── sage-sws2rst
-│ │ ├── sage_sws2rst
+│ │ ├── sage_sws2rst/
 │ │ └── setup.py
 │ └── sagemath-standard
-│ ├── bin
-│ ├── sage
+│ ├── bin/
+│ ├── sage -> ../../src/sage
 │ └── setup.py
-├── local (SAGE_LOCAL) Compiled packages are installed here
+├── local (SAGE_LOCAL) Installation hierarchy for non-Python packages
 │ ├── bin Executables
 │ ├── include C/C++ headers
-│ ├── lib Shared libraries
+│ ├── lib Shared libraries, architecture-dependent data
 │ ├── share Databases, architecture-independent data, docs
 │ │ └── doc Viewable docs of Sage and of some components
 │ └── var
-│ ├── lib/sage List of installed packages
-│ └── tmp/sage Temporary files when building Sage
+│ ├── lib/sage
+│ │ ├── installed/
+│ │ │ Records of installed non-Python packages
+│ │ ├── scripts/ Scripts for uninstalling installed packages
+│ │ └── venv-python3.9 (SAGE_VENV)
+│ │ │ Installation hierarchy (virtual environment)
+│ │ │ for Python packages
+│ │ ├── bin/ Executables and installed scripts
+│ │ ├── lib/python3.9/site-packages/
+│ │ │ Python modules/packages are installed here
+│ │ └── var/lib/sage/
+│ │ └── wheels/
+│ │ Python wheels for all installed Python packages
+│ │
+│ └── tmp/sage/ Temporary files when building Sage
 ├── logs
 │ ├── dochtml.log Log of the documentation build
 │ ├── install.log Full install log
 │ └── pkgs Build logs of individual packages
 │ ├── alabaster-0.7.12.log
 │ …
 │ └── zn_poly-0.9.2.log
-├── m4 M4 macros for configure
+├── m4 M4 macros for generating the configure script
 │ └── *.m4
 ├── Makefile Running "make" uses this file
+├── prefix -> SAGE_LOCAL Convenience symlink to the installation tree
 ├── README.md This file
 ├── sage Script to start Sage
 ├── src Monolithic Sage library source tree
-│ ├── bin  Scripts that Sage uses internally
-│ ├── doc  Sage documentation sources
-│ └── sage  The Sage library source code
+│ ├── bin/ Scripts that Sage uses internally
+│ ├── doc/ Sage documentation sources
+│ └── sage/ The Sage library source code
 ├── upstream Source tarballs of packages
 │ ├── Babel-2.9.1.tar.gz
 │ …
 │ └── zn_poly-0.9.2.tar.gz
+├── venv -> SAGE_VENV Convenience symlink to the virtual environment
 └── VERSION.txt
 ```
 For more details see [our Developer's Guide](https://doc.sagemath.org/html/en/developer/coding_basics.html#files-and-directory-structure).