Update of user-manual

The user manual has been updated with a simple explanation on how to use
Coulomb Scattering.

doc/user-manual/bibliography.bib

@@ -62,4 +62,27 @@
   publisher = {Taylor \& Francis},
 }
 
+@Article{sherlock2008monte,
+  author    = {Sherlock, Mark},
+  journal   = {Journal of Computational Physics},
+  title     = {A Monte-Carlo method for Coulomb collisions in hybrid plasma models},
+  year      = {2008},
+  number    = {4},
+  pages     = {2286--2292},
+  volume    = {227},
+  groups    = {Particle-in-cell},
+  publisher = {Elsevier},
+}
+
+@article{lemons2009small,
+  title={Small-angle Coulomb collision model for particle-in-cell simulations},
+  author={Lemons, Don S and Winske, Dan and Daughton, William and Albright, Brian},
+  journal={Journal of Computational Physics},
+  volume={228},
+  number={5},
+  pages={1391--1403},
+  year={2009},
+  publisher={Elsevier}
+}
+
 @Comment{jabref-meta: databaseType:bibtex;}