Dependencies.md

+Our aim is to keep the number of external dependencies as low as possible. However, there are a few good reasons to rely on a few external packages:
+
+## eigen3
+
+ - header only
+ - for linear algebra and geometry
+ - we provide our own version as third party
+ - avoid implicit boost dependencies: find with `grep -R "Eigen3_INCLUDE" .` and remove!
+ - use explicit boost dependence tracking with cmake target CORSIKAEigen3
+
+## boost
+
+ - header only
+ - we provide our own version as third party
+ - check the use of boost with: `grep -R "#include <boost/" .`
+ - avoid implicit boost dependencies: find with `grep -R "Boost_INCLUDE" .` and remove!
+ - use explicit boost dependence tracking with cmake target CORSIKAboost
+
+## phys/Units
+
+ - header only
+ - for physics units
+ - we provide our own version as third party
+
+## catch2
+
+ - header only
+ - for unit testing.
+ - we provide our own version as third party
+
+## pythia8
+
+ - event and decay generator
+ - we support system-level install, and provide version in ThirdParty
+
+## conex
+
+ - cascade equation solver for air showers 
+ - we support system-level install, and provide version in ThirdParty
+
+
+