Instructions to deal with 1.6.6 Boards Manager bug

See arduino/Arduino#3795

Author: per1234

Diff for: README.md

@@ -26,6 +26,7 @@ This installation method requires Arduino IDE version 1.6.4 or greater.
 - Open the **Tools > Board > Boards Manager...** menu item.
 - Wait for the platform indexes to finish downloading.
 - Scroll down until you see the **Arduino-AVRISPmkII-workaround** entry and click on it.
+- If you are using Arduino IDE 1.6.6 then you may need to close **Boards Manager** and then reopen it before the **AVRISP mkII(workaround)** entry will appear.
 - Click **Install**.
 - After installation is complete close the Boards Manager window.
 - If using Arduino IDE previous to v1.6.6 you may need to restart the IDE to make the **AVRISP mkII(workaround)** programmer appear in the **Tools > Programmer** menu.