Added 413 Request Entity Too Large to known limitations (may close #73)

Author: Zsombor Franczia

README.md

@@ -145,6 +145,8 @@ In addition `translate()` method will return boolean `FALSE` if there is no tran
 
  - `503 Service Unavailable` response:  
    If you are getting this error, it is most likely that Google has banned your external IP address and/or [requires you to solve a CAPTCHA](https://github.com/Stichoza/google-translate-php/issues/18). This is not a bug in this package. Google has become stricter, and it seems like they keep lowering the number of allowed requests per IP per a certain amount of time. Try sending less requests to stay under the radar, or change your IP frequently ([for example using proxies](#advanced-configuration)). Please note that once an IP is banned, even if it's only temporary, the ban can last from a few minutes to more than 12-24 hours, as each case is different.
+ - `413 Request Entity Too Large` response:  
+   This error means that your input string is too long. Google only allows a maximum of 5000 characters to be translated at once. If you want to translate a longer text, you can split it to shorter parts, and translate them one-by-one.
 
 ## Disclaimer