Add airspy_adsb protocol doc

protocols/airspy_adsb.md

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+# Airspy protocol
+
+## Ports
+
+* ??: server -> client feed
+
+## Format
+
+Textual format, similar to [raw](raw.md), but extended with MLAT and signal
+level data. Unlike raw format, lines are terminated with `0x0d 0x0a` (`\r\n`).
+
+
+## Frame structure
+* `0x2a` (`*`)
+* 7 or 14 byte frame (see [raw](raw.md))
+* `0x3b` (`;`)
+* 4 byte MLAT timestamp (see below)
+* `0x3b` (`;`)
+* 1 byte MLAT precision (see below)
+* `0x3b` (`;`)
+* 2 byte big-endian RSSI
+* `0x3b 0x0d 0x0a` (`;\r\n`)
+  
+
+## MLAT timestamp
+The MLAT timestamp included in each frame is the big-endian value of a counter
+at the time of packet reception. The counter runs at a rate determined by the
+precision value multiplied by 2 MHz (e.g. `0x0a` means `10 * 2 MHz = 20 MHz`).
+This counter isn't calibrated to external time, but receiving software can
+calculate its offset from other receiving stations across multiple packets, and
+then use the differences between station receive timing to calculate signal
+source position.
+
+
+## Examples
+
+* `*5DA7DA1CE30DE5;D03B5A4B;0A;7AF3;\r\n`
+* `*8DA07CD89915908778A01E4B4C86;D03D33F9;0A;8437;\r\n`
+
+
+## Implementations
+
+* [Airspy ADSB decoder](http://airspy.com/download/)