Update `.build.yml`

- `libssl-dev` is needed for newer Ubuntu versions.
- Update Racket version from 8.3 to 8.6
Use `v0.5.0` for example repo tag

Although I expect readers of the README to check the available tags,
some readers may copy/paste the Git command from the README literally.
Using the currently most recent version will give them the most features

Of course, I won't _always_ update the tag in the README, so readers of
the README should _still_ check the tags. :-)
Swap installation sections in `README.md`

I guess most people want just the command line tool, so describe its
"installation" first. ("Installation" in quotes since you only need
to download a binary ;-) )
Update `README.md`

- Add example for nested grouping.
- Fix indentation for installation instructions.
- Fix project website (omit `git.`).
Remove `git.` from project website

I use the Sourcehut project page feature now.
Update todo.txt
Improve README.md

- Make some wording clearer
- Fix Markdown formatting
Fix name of source code file

Of course, this must be the project name, not the binary name.
Mention download site for `todoreport` binary
Change version to 0.5.0
Use long option names for clarity
Run the native build binary

Or rather, run it when we can reactivate the `build-all` task (see
previous commit). ;-)
Don't run `build-all`

Due to https://github.com/racket/raco-cross/issues/7 /
the builds for Mac OS X fail.

This is a Racket bug that will be fixed only from Racket 8.4 onward, so
deactivate the build task until Racket 8.4 is released.

An alternative would be to change the makefile to skip the Mac OS X
targets, but I want to be able to run `make build-all` locally and
build for all (cs) targets.
Add `raco pkg install`

Otherwise `al2-test-runner` and `raco-exe-multitarget` won't be present.
Add `raco-exe-multitarget` as build dependency
Take project management files from sudoku-solver

Copy `VERSION`, `Makefile` and `.build.yml` from my `sudoku-solver`
package and adapt these files for `todo-txt`.
Update `README.md`

Add the new help output and provide information on the `--omit-fields`
Document `#:omit-fields` argument

Document `#:omit-fields` argument and restrictions on the combination
of completion date and creation date.
Mention restriction on date combination

Mention restriction on date combination for `make-task`. You can't use a
completion date without a creation date. See the new paragraph for
Add links to `--help` output

This is useful if users received the program as a standalone binary.