Add install graph-viz to install instructions

[librasteve]

Attempts to follow install directions are failing per this reddit thread:-

https://www.reddit.com/r/rakulang/comments/1858m14/comment/kb6wavp/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

[coke]

Addressed in d550c2e

[2colours]

@coke the fix is problematic. The file says about itself that it's a generated file - and indeed, repo_docs/README.rakudoc seems to be the source file.

[coke]

@finanalyst - what command is used to generate the markdown file from the rakudoc?

[finanalyst]

@coke Although I use GUI tool (in Pod, the terminal version should be

raku --doc=MarkDown2 doc/README.rakudoc > README.md

Pod::To::MarkDown2 is part of the Raku::Pod::Render distribution. There is a more extensive write up in that distribution's documentation.