Update README for build requirement.
Update README for build requirement.

Change-Id: Ie7680786b1e023564b8dfc51b491798123b495e7

dbrant/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,BrunoMRodrigues/apps-android-tbrc-work,SAGROUP2/apps-android-wikipedia,dbrant/apps-android-wikipedia,Wikinaut/wikipedia-app,creaITve/apps-android-tbrc-works,carloshwa/apps-android-wikipedia,Duct-and-rice/KrswtkhrWiki4Android,Wikinaut/wikipedia-app,BrunoMRodrigues/apps-android-tbrc-work,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,parvez3019/apps-android-wikipedia,Duct-and-rice/KrswtkhrWiki4Android,Wikinaut/wikipedia-app,Wikinaut/wikipedia-app,dbrant/apps-android-wikipedia,parvez3019/apps-android-wikipedia,creaITve/apps-android-tbrc-works,BrunoMRodrigues/apps-android-tbrc-work,carloshwa/apps-android-wikipedia,reproio/apps-android-wikipedia,reproio/apps-android-wikipedia,reproio/apps-android-wikipedia,reproio/apps-android-wikipedia,Duct-and-rice/KrswtkhrWiki4Android,BrunoMRodrigues/apps-android-tbrc-work,anirudh24seven/apps-android-wikipedia,parvez3019/apps-android-wikipedia,wikimedia/apps-android-wikipedia,carloshwa/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,Duct-and-rice/KrswtkhrWiki4Android,anirudh24seven/apps-android-wikipedia,carloshwa/apps-android-wikipedia,dbrant/apps-android-wikipedia,anirudh24seven/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,carloshwa/apps-android-wikipedia,Duct-and-rice/KrswtkhrWiki4Android,reproio/apps-android-wikipedia,creaITve/apps-android-tbrc-works,parvez3019/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,creaITve/apps-android-tbrc-works,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia,parvez3019/apps-android-wikipedia,SAGROUP2/apps-android-wikipedia,wikimedia/apps-android-wikipedia,Wikinaut/wikipedia-app