Adding proposal for multicast rig status/control

https://github.com/Hamlib/Hamlib/issues/695

README.multicast

@@ -0,0 +1,54 @@
+Planned for version 4.3 -- comments/suggestions about this are more than welcome
+Multicast UDP broadcast on port 4531 (one below rigctld 4532)
+Bidirectional rig control and status
+Choice of token pairs or JSON
+All packets will be tagged with ID=[unique name] -- so multiple rigs can broadcast/rx on the same port
+
+
+Broadcast packet contents to be based on get_rig_info output
+This will be the text format of name=value pairs
+Can be multiple VFO lines
+Parsers should allow for unknown tags that may be added in the future
+
+Rig command: \get_rig_info
+Rig command: \get_rig_info
+VFO=Main Freq=145000000 Mode=None Width=0 RX=1 TX=1
+VFO=VFOB Freq=145000000 Mode=FM Width=15000 RX=0 TX=0
+Split=0 SatMode=0
+PTT=1   (1=On, 0=Off)
+Rig=Dummy (name of connected rig)
+App=Hamlib (name of application providing the packet)
+Version=20210429 (version YYYYMMDD) -- reverse compatibilty will be maintained
+Status=OK   (possible values OK, Offline, Error)
+ErrorMsg=msg text (pipe delimited multi-line error message)
+CRC=0xf49f4708   (this is just an example CRC and not accurate for this example)
+
+Example JSON
+{
+  "VFO1": { // unlimited number of VFOs possible
+    "VFO": "VFOA",
+    "Freq": 14074000, // Hz
+    "Mode": "USB",
+    "Width": 5000, .. // Hz
+    "RX": 0, // boolean values 0 = off, 1 = on
+    "TX": 0,
+  },
+  "VFO2": {
+    "VFO": "VFOB",
+    "Freq": 14076000,
+    "Mode": "USB",
+    "Width": 5000,
+    "RX": 0,
+    "TX": 0,
+  },
+  "Split": 0,
+  "SatMode": 0
+  "ID": "Rig#1", // customizable rig identification
+  "Rig": "Dummy",
+  "App": "Hamlib",
+  "Version": "20210429", // protocol version date YYYYMMDD
+  "Seq": 1,  // 1-up sequence number 32-bit -- wraps around to 1 from 2^32-1
+  "CRC": "0xf49f4708" // 32-bit CRC of all data replacing the CRC value with 0x00000000
+}
+
+Will be able to set freq, mode, width, ptt