docs(README): add documentation for slotsRefreshTimeout (#487)

* Add documentation for slotsRefreshTimeout option to README

This was a new option added in redis/ioredis#475
to control timeout on cluster slots refresh.

* Typo and formatting fix in README

README.md

@@ -702,7 +702,8 @@ but a few so that if one is unreachable the client will try the next one, and th
           return delay;
         }
         ```
-    It' possible to modify the `startupNodes` property in order to switch to another set of nodes here:
+
+        It's possible to modify the `startupNodes` property in order to switch to another set of nodes here:
 
         ```javascript
         function (times) {
@@ -725,6 +726,7 @@ but a few so that if one is unreachable the client will try the next one, and th
     * `retryDelayOnTryAgain`: If this option is a number (by default, it is `100`), the client
     will resend the commands rejected with `TRYAGAIN` error after the specified time (in ms).
     * `redisOptions`: Default options passed to the constructor of `Redis` when connecting to a node.
+    * `slotsRefreshTimeout`: Milliseconds before a timeout occurs while refreshing slots from the cluster (default `1000`)
 
 ### Read-write splitting