Added documentation for the logger

documentation/logger.txt

+How it works:
+=============
+
+Whenever you want to log a message, you should specify a tag and severity, 
+for example "pci" and logger_error. The tag describes the module that is 
+responsible for the output.
+
+Filtering of messages is done by configuring the logger on load time 
+(logger::instance()->parse_configuration), the configuration specifies the 
+threshold severity level per each tag.
+
+Messages pass the filter only if their severity level is above or equal 
+what's configured.
+
+Severities: debug, info, warn, error.
+Please see pci.cc and pci.hh for usage example.
+
+Notes:
+======
+
+1. If a tag is not configured, the default severity is used: logger_error.
+2. suppressing messages - configure a tag with logger_none.
+
+What's left to do:
+==================
+
+1. Wire sprintf
+2. Configure line format (show time, show thread id, etc...)
+3. Configure using a file (change parse_configuration())
+
+Example:
+========
+
+- Log a message with the "pci" tag and debug severity:
+  logger::instance()->log("pci", logger::logger_debug, fmt("Message"));
+