doc/dev: add logging doc

Documentation/dev/logging.md

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+# Logging Conventions
+
+etcd uses the [capnslog][capnslog] library for logging application output categorized into *levels*. A log message's level is determined according to these conventions:
+
+* Error: Data has been lost, a request has failed for a bad reason, or a required resource has been lost
+  * Examples: 
+    * A failure to allocate disk space for WAL
+
+* Warning: (Hopefully) Temporary conditions that may cause errors, but may work fine. A replica disappearing (that may reconnect) is a warning.
+  * Examples:
+    * Failure to send raft message to a remote peer
+    * Failure to receive heartbeat message within the configured election timeout
+
+* Notice: Normal, but important (uncommon) log information.
+  * Examples:
+    * Add a new node into the cluster
+    * Add a new user into auth subsystem
+
+* Info: Normal, working log information, everything is fine, but helpful notices for auditing or common operations.
+  * Examples:
+    * Startup configuration
+    * Start to do snapshot
+
+* Debug: Everything is still fine, but even common operations may be logged, and less helpful but more quantity of notices.
+  * Examples:
+    * Send a normal message to a remote peer
+    * Write a log entry to disk
+
+[capnslog]: [https://github.com/coreos/pkg/tree/master/capnslog]