Create CHANGELOG.md

CHANGELOG.md

+# Changelog of postpic
+
+## v0.2.3
+2017-02-17
+
+This release brings some bugfixes and various new features.
+
+**Bugfixes**
+* Particle property Bz.
+* plotting of contourlevels.
+
+**Improvements and new features**
+* openPMD support (thanks to @ax3l).
+* ParticleHistory class to collect particle information over the entire simulation.
+* added particle properties v{x,y,z} and beta{x,y,z}.
+* Lots of performance improvemts: particle data will be much less copied in memory now.
+
+
+## v0.2.2 and earlier
+
+There hasnt been any changelog. Dont use those versions anymore.