Added StackOverflow link on README.md

I think StackOverflow is a very good platform, and helps expose Julia to a broader public. Putting a link here on the README should direct users to post more questions there.

Now, Julia is not very present at StackOverflow, and users might have the wrong impression that nobody uses the language or care about it. After all, the julia-users list is full of questions and nice answers.
Added "external resources" section, with links
Add docs for short forms of reduce and fold
Use BLAS for pairwise sumabs2 when possible
Rename varzm_pairwise -> sumabs2_pairwise

Also some minor reorganization
Make splice!(::BitVector, ::Integer, ::BitVector) return a Bool

Previously this returned a new BitArray, which was inconsistent with
splice!(::Vector{Bool}, ::Integer, ::Vector{Bool})
Update style guide to point to FAQ on type parameters
Merge pull request #6979 from JuliaLang/sjk/bitsplice

Make splice!(::BitVector, ::Integer, ::BitVector) return a Bool
Merge pull request #6985 from JuliaLang/kms/doc_param_types

Update style guide to point to FAQ on type parameters
Merge pull request #6974 from JuliaLang/sjk/faster-sumabs2

Use BLAS for pairwise sumabs2 when possible
fix #6977, trying to allocate arrayvar for Array{None}
Merge pull request #6932 from brk00/patch-1

Added StackOverflow link on README.md
fix #6966
Merge pull request #6942 from binarybana/master

Add docs for short forms of reduce and fold
make Filter iterator composable by calling done and next once per item

fixes #6943
Merge branch 'master' of https://github.com/JuliaLang/julia