Updare README.md with the Nightly build links (#305)

Author: Roberto Sora

Diff for: README.md

@@ -13,6 +13,27 @@ The Arduino Pro IDE is a completely new Development Environment for Arduino Prog
 You can download this alpha preview for testing and feedback from the [Releases Section](https://github.com/arduino/arduino-pro-ide/releases) of this repo.
 *<b>Note</b>: This is not a production release!*
 
+#### Nightly builds
+
+These builds are generated every day at 03:00 GMT from the `master` branch of the (currently private) source repository and
+should be considered unstable. In order to get the latest nightly build
+available for the supported platform, use the following links:
+
+Platform  | 32 bit                   | 64 bit                   |
+--------- | ------------------------ | ------------------------ |
+Linux     |                          | [Nightly Linux 64 bit]   |
+Windows   |                          | [Nightly Windows 64 bit] |
+macOS     |                          | [Nightly macOS 64 bit]   |
+
+[Nightly Linux 64 bit]: https://downloads.arduino.cc/arduino-pro-ide/nightly/arduino-pro-ide_nightly-latest_Linux_64bit.zip
+[Nightly Windows 64 bit]: https://downloads.arduino.cc/arduino-pro-ide/nightly/arduino-pro-ide_nightly-latest_Windows_64bit.zip
+[Nightly macOS 64 bit]: https://downloads.arduino.cc/arduino-pro-ide/nightly/arduino-pro-ide_nightly-latest_macOS_64bit.dmg
+
+> These links return an HTTP `302: Found` response, redirecting to latest
+  generated builds by replacing `latest` with the latest available build
+  date, using the format YYYYMMDD (i.e for 2019/Aug/06 `latest` is
+  replaced with `20190806` )
+
 ### Arduino CLI 
 
 The Arduino Pro IDE uses the `arduino-cli` binary found in the system's PATH or its own embedded one if the `arduino-cli` is not found.