[enh] add CHANGELOG.md

CHANGELOG.md

+## v0.50.2 (10/03/2020)
+### Code
+* Add system libs requirement and command line options in README.md
+* Fix bad label "ok" on startup message box and enhance it
+* Propose new sakia account menu at first start instead of create a new member account on the network
+* Only display licence step when registering a new member account on the network
+* Update french translation
+### Project
+* Fix release.sh using simple quote on sakia.__init__ version
+* Add this changelog
+
+## v0.50.1 (08/03/2020)
+### Code
+* Use asyncio.ensure_future() instead of async() which disappear from Py 3.7
+* Include the license file into the wheel distributed, the license changed name
+### CI/CD
+* Fix sources path and allow all file extensions to trigger the CI
+### Build
+* Add dist check after build in Makefile
+### Project
+* Fix release.sh broken by black format
+
+## [v0.50.0](https://git.duniter.org/clients/python/sakia/-/milestones/15) (07/03/2020)
+### Code
+* Update DuniterPy dependency to 0.56.0
+* Fix all detected bugs
+* Add the Sakia icon in toolbar
+* Remove search direct connections
+* Add update button in tx history
+* Fix translation system
+* Update french translation