README: macOS install instructions

[david-allison]

https://formulae.brew.sh/formula/asciidoctor

brew install asciidoctor

[flashcode635]

assign this issue to me, i am working on it

[mikehardy]

@flashcode635 this issue is trivial, could maybe take less time then it took to type your comment and for me to read it then reply here, just post a PR linked to this issue as Fixes #146

[flashcode635]

please tell me which branch to push

[flashcode635]

It is showing i am unable to access it

[flashcode635]

done,
sorry it took too much time, its my first time contributing to open source, and i am still learning about Github and open source contributions.

[mikehardy]

@flashcode635

1- fork the repository on github
2- clone the repository locally on your development machine
3- checkout a new branch for this new development
4- edit the files you want to edit
5- commit them and push to the new branch on your new fork
6- make a new PR to this repository with the changes from the branch on your fork

[flashcode635]

i have updated, please check it