mruby-cmath: add README.md

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mrbgems/mruby-cmath/README.md

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+# mruby-cmath
+
+This mrbgem provides complex number support for mruby, offering mathematical functions that can handle complex numbers, similar to Ruby's standard `cmath` library.
+
+## Requirements
+
+This gem utilizes C99 `_Complex` features. Therefore, you will need a C compiler that supports C99 or a later version to build and use this gem.
+
+## Provided Functions
+
+The `CMath` module includes the following mathematical functions:
+
+- `exp(z)`: Computes the exponential of `z`.
+- `log(z)`: Computes the natural logarithm of `z`.
+- `log(z, b)`: Computes the logarithm of `z` with base `b`.
+- `log2(z)`: Computes the base-2 logarithm of `z`.
+- `log10(z)`: Computes the base-10 logarithm of `z`.
+- `sqrt(z)`: Computes the square root of `z`.
+- `sin(z)`: Computes the sine of `z`.
+- `cos(z)`: Computes the cosine of `z`.
+- `tan(z)`: Computes the tangent of `z`.
+- `asin(z)`: Computes the arc sine of `z`.
+- `acos(z)`: Computes the arc cosine of `z`.
+- `atan(z)`: Computes the arc tangent of `z`.
+- `sinh(z)`: Computes the hyperbolic sine of `z`.
+- `cosh(z)`: Computes the hyperbolic cosine of `z`.
+- `tanh(z)`: Computes the hyperbolic tangent of `z`.
+- `asinh(z)`: Computes the inverse hyperbolic sine of `z`.
+- `acosh(z)`: Computes the inverse hyperbolic cosine of `z`.
+- `atanh(z)`: Computes the inverse hyperbolic tangent of `z`.
+
+## Usage Example
+
+Here's a simple example of how to use `CMath` to calculate the square root of a negative number:
+
+```ruby
+# Assuming mruby is built with mruby-cmath
+
+# Calculate the square root of -4
+result = CMath.sqrt(-4)
+
+# result will be a Complex number (0+2i)
+puts result.real      # Output: 0.0
+puts result.imaginary # Output: 2.0
+```