doc: added README.md (#57)

Author: sttk

README.md

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+# [cliargs][repo-url] [![Go Reference][pkg-dev-img]][pkg-dev-url] [![CI Status][ci-img]][ci-url] [![MIT License][mit-img]][mit-url]
+
+A library to parse command line arguments for Golang application.
+
+This library provides the following functionalities:
+
+- Supports [POSIX][posix-args] & [GNU][gnu-args] like short and long options.
+    - This library supports `--` option.
+    - This library doesn't support numeric short option.
+    - This library supports not `-ofoo` but `-o=foo` as an alternative to `-o foo` for short option.
+- Supports parsing with option configurations.
+- Supports parsing with a struct which stores option values and has struct tags of fields.
+- Is able to parse command line arguments including sub commands.
+- Generates help text from option configurations.
+
+
+## Import this package
+
+```
+import "github.com/sttk/cliargs"
+```
+
+## Usage
+
+The usage of this library is described on the overview in the go package document.
+
+See https://pkg.go.dev/github.com/sttk/cliargs#pkg-overview
+
+
+## Supporting Go versions
+
+This library supports Go 1.18 or later.
+
+### Actual test results for each Go version:
+
+```sh
+% gvm-fav
+Now using version go1.18.10
+go version go1.18.10 darwin/amd64
+ok  	github.com/sttk/cliargs	0.907s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	1.052s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	0.553s	coverage: 100.0% of statements
+
+Now using version go1.19.13
+go version go1.19.13 darwin/amd64
+ok  	github.com/sttk/cliargs	0.915s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	1.085s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	0.569s	coverage: 100.0% of statements
+
+Now using version go1.20.14
+go version go1.20.14 darwin/amd64
+ok  	github.com/sttk/cliargs	0.559s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	1.065s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	1.590s	coverage: 100.0% of statements
+
+Now using version go1.21.13
+go version go1.21.13 darwin/amd64
+ok  	github.com/sttk/cliargs	1.613s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	0.537s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	1.061s	coverage: 100.0% of statements
+
+Now using version go1.22.6
+go version go1.22.6 darwin/amd64
+ok  	github.com/sttk/cliargs	0.607s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	1.712s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	1.160s	coverage: 100.0% of statements
+
+Now using version go1.23.0
+go version go1.23.0 darwin/amd64
+ok  	github.com/sttk/cliargs	1.743s	coverage: 97.6% of statements
+ok  	github.com/sttk/cliargs/errors	0.599s	coverage: 100.0% of statements
+ok  	github.com/sttk/cliargs/validators	1.160s	coverage: 100.0% of statements
+
+Back to go1.22.6
+Now using version go1.22.6
+```
+
+## License
+
+Copyright (C) 2023-2024 Takayuki Sato
+
+This program is free software under MIT License.<br>
+See the file LICENSE in this distribution for more details.
+
+
+[repo-url]: https://github.com/sttk/cliargs-go
+[pkg-dev-img]: https://pkg.go.dev/badge/github.com/sttk/cliargs.svg
+[pkg-dev-url]: https://pkg.go.dev/github.com/sttk/cliargs
+[ci-img]: https://github.com/sttk/cliargs-go/actions/workflows/go.yml/badge.svg?branch=main
+[ci-url]: https://github.com/sttk/cliargs-go/actions
+[mit-img]: https://img.shields.io/badge/license-MIT-green.svg
+[mit-url]: https://opensource.org/licenses/MIT
+
+[posix-args]: https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html#Argument-Syntax
+[gnu-args]: https://www.gnu.org/prep/standards/html_node/Command_002dLine-Interfaces.html