Add content for numbers-to-indices/discrete-random-vars-binomial-dist…

…ribution

Add actual content to the skeleton of the
`numbers-to-indices/discrete-random-vars-binomial-distribution` section.

Signed-off-by: Eggert Karl Hafsteinsson <[email protected]>
Signed-off-by: Teodor Dutu <[email protected]>
Signed-off-by: Razvan Deaconescu <[email protected]>