document --with-optimizations in the README for issue26359.

README

@@ -173,10 +173,10 @@ rebuilt.  In this case, you may have to run make again to correctly
 build your desired target.  The interpreter executable is built in the
 top level directory.
 
-If you need an optimized version of Python, you type "make profile-opt"
-in the top level directory. This will rebuild the interpreter executable
-using Profile Guided Optimization (PGO). For more details, see the
-section below.
+To get an optimized build of Python, "configure --with-optimizations" before
+you run make.  This sets the default make targets up to enable Profile Guided
+Optimization (PGO) and Link Time Optimization (LTO) on most platforms.
+For more details, see the sections bellow.
 
 Once you have built a Python interpreter, see the subsections below on
 testing and installation.  If you run into trouble, see the next
@@ -194,7 +194,7 @@ Profile Guided Optimization
 ---------------------------
 
 PGO takes advantage of recent versions of the GCC or Clang compilers.
-If ran, the "profile-opt" rule will do several steps.
+If ran, "make profile-opt" will do several steps.
 
 First, the entire Python directory is cleaned of temporary files that
 may have resulted in a previous compilation.
@@ -214,6 +214,14 @@ collected in the previous one. The end result will be a Python binary
 that is optimized and suitable for distribution or production installation.
 
 
+Link Time Optimization
+----------------------
+
+LTO takes advantages of recent compiler toolchains ability to optimize across
+the otherwise arbitrary .o file boundary when building final executables or
+shared libraries for additional performance gains.
+
+
 Troubleshooting
 ---------------