[ci skip] Add readme for asynqmon tool

tools/asynqmon/README.md

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+# Asynqmon
+
+Asynqmon is a CLI tool to monitor the queues managed by `asynq` package.
+
+In order to use the tool, compile it using the following command:
+
+    go get github.com/hibiken/asynq/tools/asynqmon
+
+This will create the asynqmon executable under your `$GOPATH/bin` directory.
+
+## Quick Start
+
+Asynqmon tool has a few commands to inspect the state of tasks and queues.
+
+Run `asynqmon help` to see all the available commands.
+
+Asynqmon needs to connect to a redis-server to inspect the state of queues and tasks. Use flags to specify the options to connect to the redis-server used by your application.
+
+By default, Asynqmon will try to connect to a redis server running at `localhost:6379`.
+
+### Stats
+
+Stats command gives the overview of the current state of tasks and queues. Run it in conjunction with `watch` command to repeatedly run `stats`.
+
+Example:
+
+    watch -n 3 asynqmon stats
+
+This will run `asynqmon stats` command every 3 seconds.
+
+![Gif](/docs/assets/asynqmon_stats.gif)