add homebrew formula for macos install

[cxwx]

for example

brew install cxwx/cxbrew/jsoncons

https://github.com/cxwx/homebrew-cxbrew/blob/master/Formula/cpp/jsoncons.rb

need a somple test part for official homebrew formula.

[danielaparker]

If to support homebrew something needs to be added to the jsoncons repository, we can accept a PR. Anything related to getting jsoncons into package managers is generally done by users.