Change to accord to homie-esp8266 v2.0.0

Make OTA handling more flexible.

README.md

@@ -84,7 +84,13 @@ To efficiently parse messages, Homie defines a few rules related to topic names.
  <td>$ota</td>
  <td>Controller → Device</td>
  <td>Latest OTA version available for the device</td>
- <td>Yes</td>
+ <td>Yes or No, depending of your implementation</td>
+ </tr>
+ <tr>
+ <td>$ota/+</td>
+ <td>Controller → Device or Device → Controller</td>
+ <td>You can use any subtopics of `$ota` for anything related to your specific OTA implementation.</td>
+ <td>Yes or No, depending of your implementation</td>
  </tr>
  <tr>
  <td>$reset</td>