documentation: add quick links (#5391)

* doc: quick links

* ditto

Author: d-a-v

Diff for: README.md

@@ -1,6 +1,14 @@
 Arduino core for ESP8266 WiFi chip
 ===========================================
 
+# Quick links
+
+- [Latest release documentation](https://arduino-esp8266.readthedocs.io/en/2.4.2/)
+- [Current "git version" documentation](https://arduino-esp8266.readthedocs.io/en/latest/)
+- [Install git version](https://arduino-esp8266.readthedocs.io/en/latest/installing.html#using-git-version) ([sources](doc/installing.rst#using-git-version))
+
+# Arduino on ESP8266
+
 This project brings support for ESP8266 chip to the Arduino environment. It lets you write sketches using familiar Arduino functions and libraries, and run them directly on ESP8266, no external microcontroller required.
 
 ESP8266 Arduino core comes with libraries to communicate over WiFi using TCP and UDP, set up HTTP, mDNS, SSDP, and DNS servers, do OTA updates, use a file system in flash memory, work with SD cards, servos, SPI and I2C peripherals.