Updated Readme with rate limit warning + staging URL

There is currently no mention of Let’s Encrypt rate limits or the staging server. I’ve updated the readme so that the rate limit notice is prominently displayed and the necessary config setting to use the staging server is included in the instructions.

README.md

@@ -14,6 +14,8 @@ Current features:
 
 If you want to import existing keys from the official letsencrypt client have a look at [Import from official letsencrypt client](https://github.com/lukas2511/letsencrypt.sh/wiki/Import-from-official-letsencrypt-client).
 
+**Please note that you should use the staging URL when testing so as not to hit rate limits.** See the [Staging](#staging) section, below.
+
 Please keep in mind that this software and even the acme-protocol are relatively young and may still have some unresolved issues.
 Feel free to report any issues you find with this script or contribute by submitting a pullrequest.
 
@@ -81,6 +83,14 @@ WELLKNOWN="/var/www/letsencrypt"
 An alternative to setting the WELLKNOWN variable would be to create a symlink to the default location next to the script (or BASEDIR):
 `ln -s /var/www/letsencrypt .acme-challenges`
 
+### Staging
+
+Let’s Encrypt has stringent rate limits in place during the public beta period. If you start testing using the production endpoint (which is the default), you will quickly hit these limits and find yourself locked out. To avoid this, please set the CA property to the Let’s Encrypt staging server URL in your `config.sh` file:
+
+```bash
+CA="https://acme-staging.api.letsencrypt.org/directory"
+```
+
 ### dns-01 challenge
 
 This script also supports the new `dns-01`-type verification. This type of verification requires you to be able to create a specific `TXT` DNS record for each hostname included in the certificate.