README: link simtrace2-pysniff and note it as the recommended viewer

Links the first mention and documents it as the recommended tool for
visualising live traces and viewing saved captures, with GSMTAP pcap
import/export support.

README.md

@@ -8,9 +8,14 @@ behave like a SIMtrace2 sniffer:
 SIM card ⇄ reader (passive tap on the contacts)
     → FX2 logic analyzer (fx2lafw, 8ch, 24MHz max)
     → sigrok-cli + iso7816 decoder
-    → GSMTAP UDP :4729  →  simtrace2-pysniff / Wireshark
+    → GSMTAP UDP :4729  →  [simtrace2-pysniff](https://github.com/anttro/simtrace2-pysniff) / Wireshark
 ```
 
+[simtrace2-pysniff](https://github.com/anttro/simtrace2-pysniff) is the
+recommended tool for visualising live traces and viewing saved captures —
+it can also import/export GSMTAP pcap files (including the PCAP written by
+this decoder's `pcap_file=<path>` option).
+
 ## The tap in practice
 
 ![Components](resources/components.jpg)