Update README.md

Adding link for v0.51

Author: shaunmahony

README.md

@@ -17,6 +17,7 @@ N Yamada, PK Kuntala, BF Pugh, S Mahony. "ChExMix: a method for identifying and
 
 Downloading Executables
 --------------
+  * ChExMix version 0.51 (2021-06-03): [JAR](http://lugh.bmb.psu.edu/software/chexmix/chexmix_v0.51.jar)
   * ChExMix version 0.5 (2020-07-09): [JAR](http://lugh.bmb.psu.edu/software/chexmix/chexmix_v0.5.jar)
   * ChExMix version 0.45 (2019-12-11): [JAR](http://lugh.bmb.psu.edu/software/chexmix/chexmix_v0.45.jar)
   * ChExMix version 0.4 (2019-04-25): [JAR](http://lugh.bmb.psu.edu/software/chexmix/chexmix_v0.4.jar)