docs(README): add rate limit estimate (GeneralMills#240)

* docs(README): add rate limit estimate

* docs(README): add 429 behavior

Author: AmitMY

README.md

@@ -266,7 +266,9 @@ Returns dictionary
 * This is not an official or supported API
 * Google may change aggregation level for items with very large or very small search volume
 * Google will send you an email saying that you had a new login after running this.
-* Rate Limit is not publicly known, let me know if you have a consistent estimate.
+* Rate Limit is not publicly known, let me know if you have a consistent estimate
+  * One user reports that 1,400 sequential requests of a 4 hours timeframe got them to the limit. (Replicated on 2 networks)
+  * It has been tested, and 60 seconds of sleep between requests (successful or not) is the correct amount once you reach the limit.
 * For certain configurations the dependency lib certifi requires the environment variable REQUESTS_CA_BUNDLE to be explicitly set and exported. This variable must contain the path where the ca-certificates are saved or a SSLError: [SSL: CERTIFICATE_VERIFY_FAILED] error is given at runtime. 
 
 # Credits