docs: add how to use via nixpkgs

[kachick]

I have added the package definition in NixOS/nixpkgs#382795.
I hope this will make it easy to use by visitors without the build step. A binary cache of Nix might be an alternative solution for #69.

[reubeno]

Thanks for your contribution, @kachick, and for adding the nix package definition! I like the idea of mentioning it in README.md.

I'll review the proposed changes shortly.

[reubeno]

I'm a fan of repology; would be great to include the badge. Could you move the badge up with the rest of the badges near the top of the README? That will fit in well.

[kachick]

Sure! I have moved it with adding a blank line to fix mismatch of different badge height.

9345e82

[kachick]

This is the link for other styles of badges we can choose. https://repology.org/project/brush/badges

[reubeno]

Thanks for the info; the current badge looks fine for now.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	18.40 μs	17.91 μs	-0.49 μs	🟢 -2.69%
eval_arithmetic	0.17 μs	0.17 μs	-0.00 μs	⚪ Unchanged
expand_one_string	1.82 μs	1.97 μs	0.15 μs	⚪ Unchanged
for_loop	22.08 μs	22.05 μs	-0.03 μs	⚪ Unchanged
function_call	2.47 μs	2.80 μs	0.33 μs	⚪ Unchanged
instantiate_shell	57.00 μs	56.75 μs	-0.25 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	22496.56 μs	23190.54 μs	693.98 μs	⚪ Unchanged
parse_bash_completion	1678.78 μs	1685.56 μs	6.77 μs	⚪ Unchanged
parse_sample_script	1.74 μs	1.75 μs	0.00 μs	⚪ Unchanged
run_echo_builtin_command	15.46 μs	15.46 μs	0.00 μs	⚪ Unchanged
run_one_external_command	2073.36 μs	2432.77 μs	359.41 μs	🟠 +17.33%
tokenize_sample_script	2.82 μs	2.86 μs	0.03 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 78.27%	🟢 78.27%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 78.27%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1336	63.62
❗️ Error	234	11.14
❌ Fail	159	7.57
⏩ Skip	356	16.95
❎ Expected Fail	14	0.67
✔️ Unexpected Pass	1	0.05
📊 Total	2100	100.00

[reubeno]

Merged.

Thanks for making this change and for pursuing brush's packaging. Please don't hesitate to reach out again with any input or feedback you receive from the nix community!

[kachick]

Thank you for beginning this project!