Update README.md

[laniakea64]

• Minor reorganization to better accommodate note about using both vim-just and nvim-treesitter at the same time. Also remove instructions to use --tags flag to update git clone based installations, we never did tag anything and future tagging currently seems unlikely.
• Document that nvim-treesitter overrules vim-just by default, and how to use vim-just syntax highlighting with nvim-treesitter providing other features for justfiles. This combination seems to be desirable for some users, so let's help them avoid confusion that ends in reporting tree-sitter-just bugs here thinking it's vim-just issues.

Tag #97 (comment)