Add a nice explanation on existential-types

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

existential-types.md

@@ -1,3 +1,22 @@
 https://gist.github.com/CMCDragonkai/b203769c588caddf8cb051529339635c
 
 http://iveselov.info/posts/2012-08-30-existential-types.html
+
+Great explanation by Anupam Jain in FPNCR Telegram channel :)
+
+Everything gets a forall.
+
+Basically if you see a free variable x in a type signature, assume there’s a
+*forall x* in front of the signature.
+
+Free variable is just a fancy way to say an undeclared variable. Where *forall*
+is the way we declare variables. So if the signature is *foo :: a -> Int*,
+the *a* is free because it was not introduced first with a *forall*.
+
+*forall* at type level is like a lambda (\) at the value level. It basically
+says somehow I will be given a type and then rest of the implementation can
+depend on that type. And in fact some languages like Dhall make it explicit
+where they accept an actual type parameter instead of making it implicit
+
+In Dhall it would be something like ‘foo : a:Type -> val:a -> Int’
+It says foo takes a type, a value of that type, and gives you an Int back.