apps/system/psmq/README.txt: Add a README.txt file. This is just a copy of the description accompanying that patch that added system/psmq.

system/psmq/README.txt

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+README.txt
+==========
+
+psmq is publish subscribe message queue. It's a set of programs and libraries
+to implement publish/subscribe way of inter process communication on top of
+POSIX message queue.
+
+Manuals, source code and more info at:
+https://psmq.kurwinet.pl
+
+
+Little demo using psmqd broker, psmq_pub and psmq_sub:
+
+# start broker and make it log to file
+  nsh> psmqd -b/brok -p/sd/psmqd/psmqd.log
+
+# start subscribe thread that will read all messages send on /can/* topic
+  nsh> psmq_sub -n/sub -b/brok -t/can/* -o/sd/psmq-sub/can.log
+  n/connected to broker /brok
+  n/subscribed to /can/*
+  n/start receiving data
+
+# publish some messages
+  nsh> psmq_pub -b/brok -t/can/engine/rpm -m50
+  nsh> psmq_pub -b/brok -t/adc/volt -m30
+  nsh> psmq_pub -b/brok -t/can/room/10/temp -m23
+  nsh> psmq_pub -b/brok -t/pwm/fan1/speed -m300
+
+# check out subscribe thread logs
+  nsh> cat /sd/psmq-sub/can.log
+  [1970-01-01 00:00:53] topic: /can/engine/rpm, priority: 0, paylen: 3, payload:
+  [1970-01-01 00:00:53] 0x0000  35 30 00                                         50.
+  [1970-01-01 00:00:58] topic: /can/room/10/temp, priority: 0, paylen: 3, payload:
+  [1970-01-01 00:00:58] 0x0000  32 33 00                                         23.
+
+As you can see /adc/volt and /pwm/fan1/speed haven't been received by subscribe
+thread.
+
+Content:
+
+* psmqd - broker, relays messages between clients
+* psmq_sub - listens to specified topics, can be used as logger for
+  communication (optional)
+* psmq_pub - publishes messages directly from shell. Can send binary data, but
+  requires pipes, so on nuttx it can only send ASCII.
+* libpsmq - library used to communicate with the broker and send/receive
+  messages