README: osmo-dev: add BSC{0,1}_CODEC_LIST="fr2"

In the test setup, let the real BSC and virtual BSC use the same codec.
With this the voice frames are successfully sent back by the virtual MS.
When using a different codec, since transcoding is not implemented in
OsmoMGW, the stream becomes invalid in MGW-MSC and gets dropped by BTS1,
never reaching the virtual MS.

Related: SYS#5560
Change-Id: I78d666ad77c03459f5369b64e651e47ce1d4f12c

README.md

@@ -60,6 +60,8 @@ virtual BSC, BTS and MS (can be used next to a real BSC/BTS/MS):
 
 ```
 BSC_COUNT=2
+BSC0_CODEC_LIST="fr2"
+BSC1_CODEC_LIST="fr2"
 BTS1_RUN_IN_OSMO_DEV=1
 STP_RAN_IP="127.0.0.2"
 MS_RUN_IN_OSMO_DEV=1