SPI [FEATURE]: Add changelog file

CHANGELOG.md

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+# Changelog of SPI master and SPI slave for FPGA
+
+**Version 1.1 - released on 24 April 2021**
+- Changed license to The MIT License.
+- Added better simulations and enabled GitHub CI.
+- Added Spirit Level example design for CYC1000 board.
+- Added sync FFs to SPI slave for elimination metastability.
+- Added WORD_SIZE generic.
+- Many minor changes, fixes and optimizations.
+
+**Version 1.0 - released on 28 September 2017**
+- First non-beta release.
+- Added new version of master module with many optimizations.
+- Added DIN_LAST input to master module, for CS_N signal control.
+- Added simulation tcl script for ModelSim.
+- Updated simulation testbench.
+- Updated example design.
+- Optimized and cleaned slave module.