Update multicast JSON proposal

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

README.multicast

@@ -45,8 +45,10 @@ Example JSON
     "TX": false
   }],
 
-  "__comment_spectrum__": "Rigs that have spectrum output may include this data",
-  "Spectrum": {
+  "__comment_spectra__": "Rigs that have spectrum output may include this array data",
+  "Spectra": [
+  {
+    "Name": "Main",
     "Length": 475,
     "__comment_spectrum_data__": "2-char hex bytes so data len=2*Length",
     "Data": "00AAFF75BD2AAA...",
@@ -64,6 +66,25 @@ Example JSON
     "LowFreq": 14000000,
     "HighFreq": 14250000
   },
+  {
+    "Name": "Sub",
+    "Length": 475,
+    "__comment_spectrum_data__": "2-char hex bytes so data len=2*Length",
+    "Data": "00AAFF75BD2AAA...",
+    "Type": "FIXED|CENTER",
+    "MinLevel": 0,
+    "MaxLevel": 140,
+    "MinStrength": -100,
+    "MaxStrength": 0,
+
+    "__comment_spectrum_center__": "If Type=CENTER, the following fields will be present:",
+    "CenterFreq": 14267000,
+    "Span": 25000,
+
+    "__comment_spectrum_fixed__": "If SpectrumType=FIXED, the following fields will be present:",
+    "LowFreq": 14000000,
+    "HighFreq": 14250000
+  }],
   LastCommand {
       "ID": "MyApp 123",
       "Command": "set_freq VFOA 14074000".