docs: update dev docs

DEV.md

@@ -5,4 +5,20 @@ and take it from there. So off we go to top level main.cpp
 
 There is an original paper but I'm reading off from wiki for now https://en.wikipedia.org/wiki/Bloom_filter
 
+## Making k hashes
 
+This was the first problem. Just like open addressing hash tables do it.
+
+Idea is you take 2 hash functions and then use them as
+```
+h_i(x) = ( h1(x) + i.h2(x) ) mod m
+```
+Then if I say that `gcd(h2(x),m) = 1` then thi will loop over all residues and I have a uniform spread over the range m.
+
+I can't do h(x) + i as that's a bad idea since it'll be consecutive same with i . h(x) as there will be just equally spaced strides.
+
+This double hashed func almost behaves like a random number generator.
+
+## std::hash
+
+you make a hasher object and them immediately call it