Add doc/dependency_tree. 'cd doc/dependency_tree && tree opencoarrays' now ddepicts the OpenCoarrays dependency tree.

Signed-off-by: Damian Rouson <[email protected]>

Author: rouson

doc/dependency_tree/opencoarrays-tree

@@ -0,0 +1,9 @@
+opencoarrays/
+├── cmake-3.4.0
+└── mpich-3.1.4
+    └── gcc-5.1
+        ├── flex-2.6.0
+        │   └── bison-yacc-3.0.4
+        ├── gmp
+        ├── mpc
+        └── mpfr