doc/bips: Document BIP 125 support

doc/bips.md

@@ -18,4 +18,5 @@ BIPs that are implemented by Bitcoin Core (up-to-date up to **v0.12.0**):
 * [`BIP 66`](https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki): The strict DER rules and associated version 3 blocks have been implemented since **v0.10.0** ([PR #5713](https://github.com/bitcoin/bitcoin/pull/5713)).
 * [`BIP 70`](https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki) [`71`](https://github.com/bitcoin/bips/blob/master/bip-0071.mediawiki) [`72`](https://github.com/bitcoin/bips/blob/master/bip-0072.mediawiki): Payment Protocol support has been available in Bitcoin Core GUI since **v0.9.0** ([PR #5216](https://github.com/bitcoin/bitcoin/pull/5216)).
 * [`BIP 111`](https://github.com/bitcoin/bips/blob/master/bip-0111.mediawiki): `NODE_BLOOM` service bit added, but only enforced for peer versions `>=70011` as of **v0.12.0** ([PR #6579](https://github.com/bitcoin/bitcoin/pull/6579)).
+* [`BIP 125`](https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki): Opt-in full replace-by-fee signaling honoured in mempool and mining as of **v0.12.0** ([PR 6871](https://github.com/bitcoin/bitcoin/pull/6871)).
 * [`BIP 130`](https://github.com/bitcoin/bips/blob/master/bip-0130.mediawiki): direct headers announcement is negotiated with peer versions `>=70012` as of **v0.12.0** ([PR 6494](https://github.com/bitcoin/bitcoin/pull/6494)).