mruby-errno: add README.md

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mrbgems/mruby-errno/README.md

@@ -2,6 +2,64 @@
 
 Errno module for mruby
 
+This mrbgem provides the `Errno` module, which allows mruby programs to access system error numbers and their corresponding error messages. This is particularly useful for handling errors returned by underlying system calls.
+
+## Functionality
+
+The `mruby-errno` gem defines the `Errno` module and the `SystemCallError` class.
+
+### Accessing Error Constants
+
+The `Errno` module provides constants for various system errors. Each error constant corresponds to a specific system error number.
+
+Example:
+
+```ruby
+p Errno::EPERM::Errno  # Output: 1 (or the system-specific value for EPERM)
+p Errno::EPERM.new.message # Output: "Operation not permitted"
+```
+
+### Raising and Rescuing System Call Errors
+
+The `SystemCallError` class is the base class for system call errors. You can raise instances of `SystemCallError` or its subclasses (like `Errno::EPERM`) and rescue them in your code.
+
+Example:
+
+```ruby
+begin
+  raise Errno::EACCES, "my_method"
+rescue SystemCallError => e
+  p e.class           # Output: Errno::EACCES
+  p e.errno           # Output: 13 (or the system-specific value for EACCES)
+  p e.message         # Output: "Permission denied - my_method"
+  p e.to_s            # Output: "Permission denied - my_method"
+end
+
+# You can also rescue specific Errno constants
+begin
+  # Simulate a system call that might fail
+  # For example, trying to open a file without permissions
+  raise Errno::ENOENT, "missing_file.txt"
+rescue Errno::ENOENT => e
+  puts "Caught an ENOENT error: #{e.message}"
+  # Output: Caught an ENOENT error: No such file or directory - missing_file.txt
+end
+```
+
+### Errno::NOERROR
+
+The `Errno::NOERROR` constant represents the absence of an error, typically with a value of 0.
+
+Example:
+
+```ruby
+p Errno::NOERROR::Errno # Output: 0
+```
+
+## Defining Error Types
+
+New error types and their corresponding numbers are defined in the `known_errors.def` file. The `gen.rb` script processes this file to generate the necessary C and Ruby code for the `Errno` module. This allows `mruby-errno` to be easily extended with new system error definitions.
+
 ## License
 
 Copyright (c) 2013 Internet Initiative Japan Inc.