Summary: Currently, `--ignore-vcs` only serves to unset `--no-ignore-vcs`.
There is currently no way to tell fd to respect gitignore files when not in a
git repository.  This commit adds the flag `--no-require-git` to make fd always
respect all gitignore files.

This behaves the same as the `--no-require-git` option in [ripgrep](https://github.com/BurntSushi/ripgrep/blob/3bb71b0cb8727ac43237af78ba5c707298de0128/crates/core/app.rs#L2214-L2226)

This commit also contains an unrelated wording fix to CONTRIBUTING.md

Test Plan: `tests/tests.rs`

Background: I am using [Sapling](https://sapling-scm.com/docs/introduction/)
for working with git repositories (including this commit ☺️).  Since Sapling
uses `.sl` instead of `.git`, tools using the `ignore` crate (rg and fd) would show gitignored files.
I made a patch (https://github.com/vegerot/ripgrep/commit/ebf17eef22a04450ca86e9dc790db7cb00e11e52)
to `ignore` to respect gitignores with _either_ `.git` or `.sl`.  However,
@BurntSushi said he did not want to merge that patch and instead suggested I
use `--no-require-git` (https://github.com/BurntSushi/ripgrep/issues/2374).
This works fine, but I couldn't use this workaround for my other favorite tool!
That's what this patch is for 😁

(a follow-up patch will add a similar `FD_CONFIG_PATH` environment variable
like `RG_CONFIG_PATH`)