Added README with algorithm notes

scripts/README.md

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+# Planned algorithm
+
+From selcald:
+One possible solution to this problem is to step back from trying to determine the frequencies of the individual tones and to instead verify that:
+
+- There are two tones present
+- The difference in frequency between the two tones matches a pair of known tones in the alphabet
+
+The author of selcald seems awfully hung up on trying to ensure correct detection of the silent period
+between two tones. I don't think this is worth actively trying to detect, as it is more important
+to find two dominant tones for a period of ~1 sec, followed by another 1 sec with two dominant tones.
+The silence period is irrelevant - according to the spec of 0.2 +/- 0.1 sec _at the transmitter_ it
+could be impossibly short to reliably distinguish between successive `A*-A*` tones at the receiver.
+
+## Terrible pseudocode
+```
+collect samples from audio source <wavfile, arecord>
+low pass & decimate if necessary
+<simultaneously>
+    run wide fft over a large sample window
+    detect peak bin (with some hysteresis)
+    if peak bin within tone band:
+        assume difference is due to doppler shift
+        gradually adjust expected freq(s) for tone(s) to match
+    record RMS energy into buffer
+
+<simultaneously>
+    run correlation over small sample window
+    For each tone in the alphabet:
+        Cross correlate the audio with the adjusted tone
+        Record normalized tone energy into tone buffer
+
+    For tone record in tone buffer:
+        sort tones by amplitude
+        if:
+            - the two highest amplitude tones are within 3 dB of each other
+            - the two highest amplitude tones are at least 3 dB greater than the next nearest tone
+            - the sum of the two highest tones account for at least 60% of the modulation energy (-2.22 dB)
+                (as determined relative to stable RMS energy calc'd by wide fft)
+                (nominally 90% / -0.458 dB)
+        then:
+            record dominant two tones in tone buffer
+
+        if any two tones are dominant tones for the majority of previous 1 sec:
+            record dominant two tones in code buffer
+            reset tone detection counter (prohibit detecting codes for another 0.5 sec)
+
+    upon code record in code buffer:
+        if dominant code previously occured within 1.25 sec
+            emit SELCAL detection
+            reset code detection counter (prohibit detecting codes for another 0.5 sec)
+
+```