Clarify performance tradeoffs (#320)

(cherry picked from commit f765832)

README.rst

@@ -20,9 +20,11 @@ alternative to IOStreams.
 Features
 --------
 
-* Two APIs: faster concatenation-based write API and slower (but still
-  very fast) replacement-based format API with positional arguments for
-  localization.
+* Two APIs: faster concatenation-based `write API
+  <http://fmtlib.net/latest/api.html#write-api>`_ and slower,
+  but still very fast, replacement-based `format API
+  <http://fmtlib.net/latest/api.html#format-api>`_ with positional arguments
+  for localization.
 * Write API similar to the one used by IOStreams but stateless allowing
   faster implementation.
 * Format API with `format string syntax
@@ -35,7 +37,7 @@ Features
 * Support for user-defined types.
 * High speed: performance of the format API is close to that of
   glibc's `printf <http://en.cppreference.com/w/cpp/io/c/fprintf>`_
-  and better than performance of IOStreams. See `Speed tests`_ and
+  and better than the performance of IOStreams. See `Speed tests`_ and
   `Fast integer to string conversion in C++
   <http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_.
 * Small code size both in terms of source code (format consists of a single

doc/api.rst

@@ -22,6 +22,11 @@ arguments in the resulting string.
 
 *args* is an argument list representing arbitrary arguments.
 
+The `performance of the format API
+<https://github.com/fmtlib/fmt/blob/master/README.rst#speed-tests>`_ is close
+to that of glibc's ``printf`` and better than the performance of IOStreams.
+For even better speed use the `write API`_.
+
 .. _format:
 
 .. doxygenfunction:: format(CStringRef, ArgList)
@@ -136,6 +141,13 @@ a POSIX extension for positional arguments.
 Write API
 =========
 
+The write API provides classes for writing formatted data into character
+streams. It is usually faster than the `format API`_ but, as IOStreams,
+may result in larger compiled code size. The main writer class is
+`~fmt::BasicMemoryWriter` which stores its output in a memory buffer and provides
+direct access to it. It is possible to write create custom writers that
+store output elsewhere by subclassing `~fmt::BasicWriter`.
+
 .. doxygenclass:: fmt::BasicWriter
    :members: