Add installation instructions for gfmrun

[jllovet]

What type of PR is this?

• Documentation

What this PR does / why we need it:

Contributor docs did not include that gfmrun is a dependency for running make all. The PR adds a few lines with instructions on doing that.

Special notes for your reviewer:

This is my first PR for this repo. Any suggestions - or a correction if I'm off base here - would be helpful.

Testing

On a machine that does not havegfmrun, run:

git clone https://github.com/urfave/cli.git

Run

make

Final steps should fail.

Now run installation described in PR:

go install github.com/urfave/gfmrun/cmd/gfmrun@latest

Re-run

make

Final steps should pass.

Release Notes

NONE

[meatballhat]

I think I'd like to address this documentation bug a bit differently since the ensure-gfmrun command is meant to solve the problem and is what's used in CI:

cli/scripts/build.go

Lines 98 to 101 in 3976dfc

	{
	Name: "ensure-gfmrun",
	Action: EnsureGfmrunActionFunc,
	},

cli/.github/workflows/publish-docs.yml

Line 33 in 3976dfc

- run: make ensure-gfmrun

[jllovet]

@meatballhat's suggestion seems reasonable to me, based on my understanding of build.go. gfmrun will get installed by EnsureGfmrunActionFunc if it's missing. Should we refashion this portion of the docs to suggest that a user run make ensure-gfmrun directly? While we're at it, are there other things that would be good to check or run for someone trying to contribute?