Extend `Makefile`

[cyqsimon]

I've updated the instructions in README.md to use make. This will make building from source somewhat easier for the user (and for packagers like me).

It now also installs shell completion scripts and man pages, which were previously left out of the instructions.

[alerque]

This is certainly an improvement, but it is also doing quite a bit of reinventing the wheel doing a lot of what autotools could/would do. I realize it's a bit obtuse so the answer might be "because I don't know how to build a Rust project with autotools", but if platform support, consistency, and doing the expected thing when installing is desired I'd be happy to contribute the autotools tooling to make that happen. As an example of using autotools to build using Cargo and install completions and such my git-warp-time project is a pretty basic example.

[cyqsimon]

@alerque yeah I'm not familiar with autotools at all, and if properly done I'm sure it'll be better than this. If you want to give it a shot go ahead, all I want is to have this whole installation routine simplified.

[alerque]

I'm not sure switching to autotools would count a "simplification", but it would increase the robustness. The surface area that a maintainer would need to touch to, say, add or remove bits from the package would be about the same, but the overall level of tooling complexity would go up a bit. The benefit is that it would work more consistently across platforms as expected and lots of edge cases with paths and dependencies and weird shells and such would be smoothed over so ./configure && make && make install would just work out of the box on more systems.

I guess it's going to be up to @kpcyrd. I can probably contribute the autotools conversion using this as a starting point if that's something they want to see that direction taken for this project.