Add URL to ppls1 to README.md

examples/DropletCollision/README.md

@@ -4,7 +4,7 @@
 Two spherical liquid droplets (LJTS) are set in vapor next to each other. A macroscopic velocity was assigned to them so that they will collide eventually.
 
 ## Setup
-The provided script will setup the initial checkpoint including the two droplets. To make the script run, make sure that the ls1 python toolbox ppls1 works properly. Change the paths in the python script according to the present system.
+The provided script will setup the initial checkpoint including the two droplets. To make the script run, make sure that the ls1 python toolbox [ppls1](https://github.com/ls1mardyn/ls1-mardyn/tree/master/tools/ppls1/ppls1) works properly. Change the paths in the python script according to the present system.
 
 ## Steps
 The script will execute three steps, represented in the specified working folder as `s1_Bulk-Liquid`, `s2_Droplet-Equi`, and `s3_Droplet_Collision`.