Update the README to mention that a C API is also available.

README

 Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 The Chrome OS "metrics" package contains utilities for client-side
 user metric collection. The collected data is sent to Chrome for
 transport to the UMA server.
 
 
 ================================================================================
 The Metrics Library: libmetrics
 ================================================================================
 
-libmetrics is a small library that implements the basic C++ API for
-metrics collection. All metrics collection is funneled through this
-library. The easiest and recommended way for a client-side module to
-collect user metrics is to link libmetrics and use its APIs to send
+libmetrics is a small library that implements the basic C and C++ API
+for metrics collection. All metrics collection is funneled through
+this library. The easiest and recommended way for a client-side module
+to collect user metrics is to link libmetrics and use its APIs to send
 metrics to Chrome for transport to UMA. In order to use the library in
 a module, you need to do the following:
 
 - Add a dependence (DEPEND and RDEPEND) on chromeos-base/metrics to
   the module's ebuild.
 
 - Link the module with libmetrics (for example, by passing -lmetrics
   to the module's link command). Both libmetrics.so and libmetrics.a
   are built and installed under $SYSROOT/usr/lib/. Note that by
   default -lmetrics will link against libmetrics.so, which is
@@ skipping 12 matching lines @@
 
   bool MetricsLibrary::SendEnumToUMA(const std::string& name, int sample,
                                      int max)
   sends a sample for an enumeration (linear) histogram.
 
   Before using these methods, a MetricsLibrary object needs to be
   constructed and initialized through its Init method. See the
   complete API documentation in metrics_library.h under
   src/platform/metrics/.
 
+  For more information on the C API see c_metrics_library.h.
+
 - On the target platform, shortly after the sample is sent it should
   be visible in Chrome through "about:histograms".
 
 
 ================================================================================
 Histogram Naming Convention
 ================================================================================
 
 Use TrackerArea.MetricName. For example:
 
 Logging.CrashCounter
 Network.TimeToDrop
-Platform.BootTime
 
 
 ================================================================================
 Server Side
 ================================================================================
 
 If the histogram data is visible in about:histograms, it will be sent
 by an official Chrome build to UMA, assuming the user has opted into
 metrics collection. To make the histogram visible on
 "chromedashboard", the histogram wiki needs to be updated (steps 2 and
@@ skipping 35 matching lines @@
 screen saver states to determine if the user is actively using the
 device or not and generates the corresponding data. The metrics daemon
 uses libmetrics to send the data to Chrome.
 
 The recommended way to generate metrics data from a module is to link
 and use libmetrics directly. However, the module could instead send
 signals to or communicate in some alternative way with the metrics
 daemon. Then the metrics daemon needs to monitor for the relevant
 events and take appropriate action -- for example, aggregate data and
 send the histogram samples.