Add shell completions and README to build script

[jdsutherland]

Fixes #508. Possibly requires changes to facilitate #394? Additionally, might want to make relevant deletions from https://github.com/exercism/cli.exercism.io per exercism/DEPRECATED.cli-www#34?

Completion scripts currently live in the CLI website repository which is
deemed inappropriate

Update build script to add shell completions and README to tar/zips

[jdsutherland]

From #394:

I need to figure out how to tell Homebrew where to put the auto-complete scripts, or if it has some good defaults.

I have no prior experience with the homebrew repo but it seems https://github.com/Homebrew/homebrew-core/blob/c53141dcda9d1a062eba89f77a5c02f09f6b5bd2/Formula/fzf.rb#L52 + fzf/shell might help.

[kytrinyx]

Thank you! This will be such a relief to get done.

[kytrinyx]

Let's keep the scripts in this repository instead of in the static HTML repo (as you suggested). That way this will be a simple file inclusion rather than a network call.

[kytrinyx]

I like the direction of this. A few things:

• the build doesn't bail, but the resulting artifacts won't extract because it contains ../
• let's move the BUILD.md into shell/ and rename it to README.md

[kytrinyx]

Here's the error I got when I tried to extract the archive:

$ tar -xvzf exercism-mac-64bit.tgz 
x exercism
x ../shell/: Path contains '..'
x ../shell/exercism_completion.bash: Path contains '..'
x ../shell/exercism_completion.zsh: Path contains '..'
x ../BUILD.md: Path contains '..'
tar: Error exit delayed from previous errors.

[kytrinyx]

FYI: I changed the base of this PR to point to master (I've been changing branches around, so what is on master now is what was previously on nextercism).

[kytrinyx]

❤️ 💛 💚 💙 💜 🖤 thanks for this!