Add to readme example code to stop loading packer if requirements not met

[eduardoarandah]

When packer is first loaded and some requirements aren't there, it explodes.

A simple check for requirements would be nice. So people can copy paste it.

and maybe add some examples for some popular plugins:

• I think treesitter requires some c compiler
• telescope uses ripgrep

quick draft:

-- Check requirements
local all_requirements = 1
for _, requirement in ipairs({ "git", "node", "npm" }) do
	if vim.fn.executable(requirement) == 0 then
		print("plugins require " .. requirement)
		all_requirements = 0
	end
end

if all_requirements == 0 then
	print("can't install plugins")
	return
end