README: Use h3 headings for build sections, and add TOC (GH-64)

Author: eklitzke

README.rst

@@ -40,6 +40,7 @@ Installable Python kits, and information about using Python, are available at
 
 .. _python.org: https://www.python.org/
 
+.. contents::
 
 Build Instructions
 ------------------
@@ -85,7 +86,7 @@ below.
 
 
 Profile Guided Optimization
----------------------------
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 PGO takes advantage of recent versions of the GCC or Clang compilers.  If ran,
 ``make profile-opt`` will do several steps.
@@ -109,7 +110,7 @@ optimized and suitable for distribution or production installation.
 
 
 Link Time Optimization
-----------------------
+^^^^^^^^^^^^^^^^^^^^^^
 
 Enabled via configure's ``--with-lto`` flag.  LTO takes advantage of the
 ability of recent compiler toolchains to optimize across the otherwise