Added keyfile property reminder related to stripe for production releases

cmsWebApp/webroot/extensions/performa/upgrades/20180116_Add_StripeProperties.txt

+Add followings to keyfile.properties - only for production
+
+stripe.key=sk_live_oqwyeVyRa7Cpy2XmEgbTmi8D
+stripe.pubkey=pk_live_pxpYJQUaXO4aYiBIy03GQA8D
+
+Following are required only if stripe plan and coupon details have changed. Default values set are 0001 and EAP respectively.
+
+stripe.plan.id
+stripe.coupon.id
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