<docs> update en doc

docs/src/log/contents.rst

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+.. _log:
+
+Running log
+===========
+
+.. toctree::
+
+   log_file
+   log_verbose

docs/src/log/log_file.rst

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+.. _log_file:
+
+Enable log file
+================
+
+.. tip::
+
+  If import glog to build.
+
+The general configuration of the log in the head file `logger.h <https://github.com/slightech/MYNT-EYE-S-SDK/blob/master/include/mynteye/logger.h>`_ .
+
+Uncomment ``FLAGS_log_dir = \".\";`` recompile and save to current work directory. Run ``camera_a`` log file as follows:
+
+.. code-block:: none
+
+  <workdir>/
+  ├─camera_a.ERROR
+  ├─camera_a.FATAL
+  ├─camera_a.INFO
+  ├─camera_a.WARNING
+  ├─camera_a.john-ubuntu.john.log.ERROR.20180513-141833.519
+  ├─camera_a.john-ubuntu.john.log.FATAL.20180513-141833.519
+  ├─camera_a.john-ubuntu.john.log.INFO.20180513-141832.519
+  └─camera_a.john-ubuntu.john.log.WARNING.20180513-141833.519
+
+``camera_a.INFO`` shows the program and levers of log it is running. The link to the real log file is ``camera_a.john-ubuntu.john.log.INFO.20180513-141832.519``. Even if it ran several times, ``camera_a.INFO`` still leaves the link to last log file.
+
+Excute `make cleanlog` to clean all log files.

docs/src/log/log_verbose.rst

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+.. _log_verbose:
+
+Enabled detailed level
+=======================
+
+.. tip::
+
+  If import glog to build.
+
+The general configuration of the log is in the head file `logger.h <https://github.com/slightech/MYNT-EYE-S-SDK/blob/master/include/mynteye/logger.h>`_ .
+
+Uncomment ``FLAGS_v = 2`` ; and recompile to enable the detail levels, the log is printed by ``VLOG(n)``
+
+For information on how to use the log library, such as how to configure, print, etc., please open its document and learn more:
+
+.. code-block:: bash
+
+  $ ./scripts/open.sh third_party/glog/doc/glog.html