added note to dns

after i run into issues https://github.com/Neilpang/acme.sh/issues/225 - just because i was to fast - i modified the Readme that people should be patient about DNS.

Hope that prevents someone running into the same issues.

README.md

@@ -219,8 +219,12 @@ Add the following txt record:
 Domain:_acme-challenge.www.aa.com
 Txt value:9ihDbjxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 
+```
+
 Please add those txt records to the domains. Waiting for the dns to take effect.
 
+```
+If you did not wait until DNS knows your values the renew will fail, be patient!
 ```
 
 Then just rerun with `renew` argument: