Update modifications list

Author: kelvin-chappell

modifications.md

@@ -1,13 +1,17 @@
 # Guardian-specific modifications
 These are the ways in which the Guardian optimised build differs from the [generic Prebid](https://github.com/prebid/Prebid.js) build:
+## General
 * Ad server targeting includes a `hb_ttr` parameter, whose value will be one of:
     * the time to respond for the winning bid in ms (ie. time between bid request sent and bid response received)
     * -1 if the auction timed out without a winning bid and still waiting for at least one bid response 
     * otherwise not passed at all
-* We have built our own custom [Index Exchange bid adapter](https://github.com/guardian/Prebid.js/blob/master/modules/guIndexExchangeBidAdapter.js) 
-* The [Sonobi bid adapter](https://github.com/guardian/Prebid.js/blob/master/modules/sonobiBidAdapter.js):
+## Bid adapters
+* We have built our own custom [Index Exchange adapter](https://github.com/guardian/Prebid.js/blob/master/modules/guIndexExchangeBidAdapter.js) 
+* The [Sonobi adapter](https://github.com/guardian/Prebid.js/blob/master/modules/sonobiBidAdapter.js):
     * has an extra request parameter, `gmgt`, holding AppNexus targeting key-values
     * has a customised `pv` parameter, holding the Ophan-generated pageview ID
+* The [AppNexus adapter](https://github.com/guardian/Prebid.js/blob/master/modules/appnexusBidAdapter.js) has an alias `xhb` for Xaxis
+## Analytics adapters
 * We have built two analytics adapters:
     * an [adapter](https://github.com/guardian/Prebid.js/blob/master/modules/guAnalyticsAdapter.js) to send analytics to the data lake
     * a simple console-logging [adapter](https://github.com/guardian/Prebid.js/blob/master/modules/consoleLoggingAnalyticsAdapter.js)