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

README

@@ -46,16 +46,17 @@ For example:
 (This will fail if you *also* built at the top-level directory.
 You should do a "make clean" at the toplevel first.)
 
-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 bellow.
+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.
 
 
 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.
@@ -75,6 +76,14 @@ 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.
+
+
 What's New
 ----------