update index of learn.md

The index of booklets in `learn.md` was amended by a link about
the new booklet under construction.

Signed-off-by: Norwid Behrnd <[email protected]>

Author: nbehrnd

source/learn.md

@@ -186,4 +186,5 @@ learn/best_practices/index
 learn/intrinsics/index
 learn/rosetta_stone
 learn/oop_features_in_fortran/index
+learn/f95_features/index
 :::