Add README

Author: bblanchon

HeapFragmentation/README.md

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+What is heap fragmentation?
+===========================
+
+An Arduino sketch that demonstrate the increase of heap fragmentation.
+
+[Read the articule on cpp4arduino.com](https://cpp4arduino.com/2018/11/06/what-is-heap-fragmentation.html)
+
+### Supported platforms
+
+Because it inspects the content of the heap, this program highly depends on
+the "core" of the board.
+
+Only the following cores are supported:
+
+* [The official Arduino AVR core](https://github.com/arduino/ArduinoCore-avr) (bundled with the IDE):
+    - Adafruit Circuit Playground
+    - Arduino BT
+    - Arduino Duemilanove or Diecimila
+    - Arduino Esplora
+    - Arduino Ethernet
+    - Arduino Fio
+    - Arduino Gemma
+    - Arduino Industrial 101
+    - Arduino Leonardo
+    - Arduino Leonardo ETH
+    - Arduino Mega ADK
+    - Arduino Mini
+    - Arduino Nano
+    - Arduino NG or older
+    - Arduino Pro or Pro Mini
+    - Arduino Robot Control
+    - Arduino Robot Motor
+    - Arduino Uno WiFi
+    - Arduino Yún
+    - Arduino Yún Mini
+    - Arduino/Genuino Mega or Mega 2560
+    - Arduino/Genuino Micro
+    - Arduino/Genuino Uno
+    - LilyPad Arduino
+    - LilyPad Arduino USB
+    - Linino One
+* [The ESP8266 core for Arduino](https://github.com/esp8266/Arduino):
+    - 4D Systems gen4 IoD Range
+    - Adafruit Feather HUZZAH ESP8266
+    - Amperka WiFi Slot
+    - Arduino
+    - Digistump Oak
+    - ESPDuino (ESP-13 Module)
+    - ESPino (ESP-12 Module)
+    - ESPresso Lite 1.0
+    - ESPresso Lite 2.0
+    - Generic ESP8266 Module
+    - Generic ESP8285 Module
+    - Invent One
+    - LOLIN(WEMOS) D1 mini Lite
+    - LOLIN(WEMOS) D1 mini Pro
+    - LOLIN(WEMOS) D1 R2 & mini
+    - NodeMCU 0.9 (ESP-12 Module)
+    - NodeMCU 1.0 (ESP-12E Module)
+    - Olimex MOD-WIFI-ESP8266(-DEV)
+    - Phoenix 1.0
+    - Phoenix 2.0
+    - Seeed Wio Link
+    - SparkFun ESP8266 Thing
+    - SparkFun ESP8266 Thing Dev
+    - SweetPea ESP-210
+    - ThaiEasyElec's ESPino
+    - WeMos D1 R1
+    - WiFiduino
+    - WifInfo
+    - XinaBox CW01
+
+Adding support for other core requires adding a new variant in
+[the folder "Ports"](Ports/) and to include the file in
+[MemoryInfo.cpp](MemoryInfo.cpp).