v1.4.0 to fix multiple-definitions linker error

### Releases v1.4.0

1. Fix `multiple-definitions` linker error. Drop `src_cpp` and `src_h` directories
2. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project.
3. Fix `reattachInterrupt()` bug. Check [bugfix: reattachInterrupt() pass wrong frequency value to setFrequency() #19](khoih-prog/ESP8266TimerInterrupt#19)
4. Optimize library code by using `reference-passing` instead of `value-passing`
5. Update all examples

README.md

@@ -18,6 +18,7 @@
   * [Important notes about NRF_TIMER_1](#important-notes-about-nrf_timer_1)
   * [Currently supported Boards](#currently-supported-boards)
   * [Important Notes about ISR](#important-notes-about-isr)
+* [Changelog](changelog.md) 
 * [Prerequisites](#prerequisites)
 * [Installation](#installation)
   * [Use Arduino Library Manager](#use-arduino-library-manager)