mruby-strftime: add readme documentation

add comprehensive readme covering usage, format specifiers,
features, and implementation details.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-strftime/README.md

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+# mruby-strftime
+
+`Time#strftime` implementation for mruby.
+
+## Overview
+
+This gem provides the `strftime` method for `Time` objects in mruby, enabling
+formatted time string output using standard format directives.
+
+## Usage
+
+```ruby
+require 'mruby-strftime'
+
+t = Time.new(2023, 12, 25, 10, 30, 45)
+
+t.strftime("%Y-%m-%d")           #=> "2023-12-25"
+t.strftime("%H:%M:%S")           #=> "10:30:45"
+t.strftime("%Y-%m-%d %H:%M:%S")  #=> "2023-12-25 10:30:45"
+t.strftime("%A, %B %d, %Y")      #=> "Monday, December 25, 2023"
+```
+
+## Format Specifiers
+
+The `strftime` method supports standard format directives. The exact set of
+available directives depends on your system's `strftime(3)` implementation.
+Common directives include:
+
+### Date Components
+
+- `%Y` - Year with century (e.g., 2023)
+- `%y` - Year without century (00-99)
+- `%m` - Month of the year (01-12)
+- `%B` - Full month name (e.g., "December")
+- `%b` - Abbreviated month name (e.g., "Dec")
+- `%d` - Day of the month (01-31)
+- `%j` - Day of the year (001-366)
+
+### Time Components
+
+- `%H` - Hour of the day, 24-hour clock (00-23)
+- `%I` - Hour of the day, 12-hour clock (01-12)
+- `%M` - Minute of the hour (00-59)
+- `%S` - Second of the minute (00-60)
+- `%p` - AM/PM indicator
+- `%Z` - Timezone name or abbreviation
+
+### Weekday
+
+- `%A` - Full weekday name (e.g., "Monday")
+- `%a` - Abbreviated weekday name (e.g., "Mon")
+- `%w` - Day of the week (0-6, Sunday is 0)
+- `%u` - Day of the week (1-7, Monday is 1)
+
+### Combined Formats
+
+- `%c` - Preferred date and time representation
+- `%x` - Preferred date representation
+- `%X` - Preferred time representation
+- `%F` - ISO 8601 date format (equivalent to `%Y-%m-%d`)
+- `%T` - ISO 8601 time format (equivalent to `%H:%M:%S`)
+
+### Special Characters
+
+- `%%` - Literal `%` character
+- `%n` - Newline character
+- `%t` - Tab character
+
+## Features
+
+### Timezone Support
+
+The method respects the timezone of the `Time` object:
+
+```ruby
+t_utc = Time.utc(2023, 12, 25, 12, 0, 0)
+t_local = Time.local(2023, 12, 25, 12, 0, 0)
+
+t_utc.strftime("%Y-%m-%d %H:%M:%S %Z")
+#=> "2023-12-25 12:00:00 UTC"
+
+t_local.strftime("%Y-%m-%d %H:%M:%S %z")
+#=> "2023-12-25 12:00:00 +0900" (example for JST)
+```
+
+### NUL Byte Handling
+
+Unlike some implementations, mruby-strftime correctly handles NUL bytes (`\0`)
+embedded in format strings, preserving them in the output:
+
+```ruby
+t = Time.gm(2023, 12, 25)
+result = t.strftime("year\0%Y")
+result.length  #=> 9 (includes the NUL byte)
+```
+
+This behavior maintains Ruby's string semantics where strings are length-based
+rather than null-terminated.
+
+## Implementation Details
+
+### Buffer Management
+
+The implementation uses dynamic buffer allocation:
+
+- Initial buffer size: 64 bytes
+- Maximum buffer size: 4096 bytes
+- Automatically grows when needed for longer formatted strings
+
+### Platform Compatibility
+
+This gem uses the system's `strftime(3)` function for formatting. The exact
+behavior of format directives may vary slightly between platforms, particularly
+for:
+
+- Locale-dependent formats (day/month names, date/time preferences)
+- Timezone representations
+- Platform-specific extensions
+
+## Dependencies
+
+- `mruby-time` - Required for `Time` class support
+
+## Installation
+
+Add to your `build_config.rb`:
+
+```ruby
+conf.gem :core => 'mruby-strftime'
+```
+
+Or include via gembox:
+
+```ruby
+conf.gembox 'stdlib-ext'  # Includes mruby-strftime
+```
+
+## License
+
+MIT License - See mruby's main license file for details.