Update readme

[FreezyLemon]

After seeing that the README was pretty outdated, I decided to try and update it a bit. Some things I'm not 100% about:

• The first few sentences. These are pretty important as far as READMEs go, so feel free to edit/propose stuff
• Saying lsmash is the recommended choice. This just seemed like the generally preferred choice from what I read in Discord
• I think "use av1an --help for a CLI reference" is easier to maintain than listing every option in the readme
• Where to put the image of the 96-worker encode

Feel free to give feedback on these things and in general.

[redzic]

Overall I think this looks very good. I do wonder if we could also add some instructions for how to install lsmash and/or ffms2 on Windows. Basically the user just needs to put some DLL files into their vapoursynth plugins directory, but most users probably wouldn't know that in addition to knowing where to find the DLLs.

[jangrewe]

I would actually prefer to have the full --help output in the README, as an easy reference without having the run the command and scroll back and forth in the console all the time. It doesn't need to be nicely formatted, so putting the raw output into a code block would be good enough.

[FreezyLemon]

@jangrewe I don't mind including the --help output somewhere, and spending some time on formatting isn't really an issue either. The main reason that I removed the old "parameter block" from the readme was the lack of maintenance.

There was a lot of missing and some wrong information, so I thought it would be better to just tell people to use av1an --help instead of referencing the outdated information.

It's no real problem though, if you prefer a parameter reference that doesn't required running av1an then I can include it.

[FreezyLemon]

Well, I think with the last commit I'm pretty much done for now. Feels a bit bad to leave the image at the bottom, but it doesn't really fit anywhere else.

[master-of-zen]

I think preview image should be on top

[redzic]

Looks good, thanks!