supersize: Update README.md with technical details.

tools/binary_size/libsupersize/models.py

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Classes that comprise the data model for binary size analysis.
 
 The primary classes are Symbol, and SymbolGroup.
 
 Description of common properties:
   * address: The start address of the symbol.
         May be 0 (e.g. for .bss or for SymbolGroups).
@@ skipping 337 matching lines @@
       if self.IsBss():
         self._size = sum(s.size for s in self)
       else:
         self._size = sum(s.size for s in self.IterUniqueSymbols())
     return self._size
 
   @property
   def pss(self):
     if self._pss is None:
       if self.IsBss():
-        self._pss = float(self.size)
+        self._pss = sum(s.pss for s in self)
       else:
-        self._pss = sum(s.pss for s in self)
+        self._pss = sum(s.pss for s in self if not s.IsBss())
     return self._pss
 
   @property
   def padding(self):
     if self._padding is None:
       self._padding = sum(s.padding for s in self.IterUniqueSymbols())
     return self._padding
 
   @property
   def aliases(self):
@@ skipping 370 matching lines @@
 
 
 def _ExtractSuffixAfterSeparator(string, separator, count):
   prev_idx = len(string) + 1
   for _ in xrange(count):
     idx = string.rfind(separator, 0, prev_idx - 1)
     if idx < 0:
       break
     prev_idx = idx
   return string[:prev_idx]