Add backwards compatibility section

Author: shaolinfry

bip-uaversionbits.mediawiki

@@ -55,6 +55,10 @@ The state transition workflow is exactly the same as in [[bip-0009.mediawiki|BIP
 
 https://github.com/bitcoin/bitcoin/compare/master...shaolinfry:bip-uaversionbits
 
+==Backwards compatibility==
+
+If '''lockinontimeout''' boolean is set to false this BIP is compatible with nodes that implement BIP9. If '''lockinontimeout''' boolean is set to true, nodes that only implement BIP9 will not activate the soft fork if hashpower threshold is not reached by '''timeout''', however, those nodes will still accept the blocks generated by activated nodes.
+
 ==Deployments==
 
 A living list of deployment proposals can be found [[bip-0009/assignments.mediawiki|here]].