gluon-airtime: Add readme

package/gluon-airtime/README.md

@@ -0,0 +1,36 @@
+This module adds a respondd airtime usage statistics provider.
+The format is the following:
+
+```json
+{
+  "statistics": {
+    "wireless": [
+      {
+        "frequency": 5220,
+        "active": 366561161,
+        "busy": 46496566,
+        "rx": 808415,
+        "tx": 41711344,
+        "noise": 162
+      },
+      {
+        "frequency": 2437,
+        "active": 366649704,
+        "busy": 205221222,
+        "rx": 108121446,
+        "tx": 85453679,
+        "noise": 161
+      }
+    ]
+  }
+}
+```
+
+The numbers `active`, `busy`, `rx` and `tx` are times in milliseconds, where
+`busy`, `rx` and `tx` have to be interpreted by taking the quotient with
+`active`.
+
+The motivation for having a list with the frequency as a value in the objects
+instead of having an object with the frequency as keys is that multiple wifi
+devices might be present, in which case the same frequency can appear multiple
+times (because the statistics are reported once for every phy).