mruby-rational: add README.md

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mrbgems/mruby-rational/README.md

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+# mruby-rational
+
+This mrbgem provides a `Rational` class for mruby, allowing you to work with rational numbers (fractions).
+
+## Usage
+
+To use the `Rational` class, you first need to include the mrbgem in your mruby build.
+
+### Include in build_config.rb
+
+Add the following line to your `build_config.rb` file:
+
+```ruby
+conf.gem :core => 'mruby-rational'
+```
+
+### Creating Rational Objects
+
+You can create `Rational` objects using the `Rational()` method:
+
+```ruby
+r1 = Rational(1, 2)  # Represents 1/2
+r2 = Rational(3, 4)  # Represents 3/4
+```
+
+### Arithmetic Operations
+
+The `Rational` class supports standard arithmetic operations:
+
+```ruby
+r_add = Rational(1, 2) + Rational(1, 3)  # (5/6)
+r_sub = Rational(1, 2) - Rational(1, 3)  # (1/6)
+r_mul = Rational(1, 2) * Rational(1, 3)  # (1/6)
+r_div = Rational(1, 2) / Rational(1, 3)  # (3/2)
+```
+
+## Available Methods
+
+The `Rational` class provides the following important methods:
+
+- `numerator`: Returns the numerator of the rational number.
+- `denominator`: Returns the denominator of the rational number.
+- `to_f`: Converts the rational number to a `Float`.
+- `to_i`: Converts the rational number to an `Integer` (truncates towards zero).
+- `to_s`: Returns a string representation of the rational number (e.g., "1/2").
+- `inspect`: Returns a string representation suitable for debugging (e.g., "(1/2)").
+- `==`: Checks for equality with another number.
+- `<=>`: Compares the rational number with another number.
+- `positive?`: Returns `true` if the rational number is greater than zero.
+- `negative?`: Returns `true` if the rational number is less than zero.
+
+## Error Handling
+
+The mrbgem handles common errors such as:
+
+- **DivisionByZeroError**: Raised when attempting to create a rational number with a denominator of zero.
+- **RangeError**: Raised in case of integer overflow during calculations.
+
+## License
+
+mruby-rational is licensed under the MIT License. See LICENSE for details.