add attribute reference to readme

Author: panicbit

README.md

@@ -2,9 +2,9 @@
 
 Derive `Debug` with a custom format per field.
 
-# Usage
+# Example usage
 
-Here is a showcase of all possible field attributes:
+Here is a showcase of `custom_debug`s features:
 
 ```rust
     use custom_debug::Debug;
@@ -34,4 +34,22 @@ Foo {
     x: 42 things,
     z: 0xAB
 }
-```
+```
+
+# Field attributes reference
+
+Attributes within a section below are considered mutually exclusive.
+
+## Skip attributes
+
+| | |
+|-|-|
+| `skip` | Unconditionally skips a field. |
+| `skip_if = path::to::function` | Skips a field if `path::to::function(&field)` returns `true`. |
+
+## Format attributes
+
+| | |
+|-|-|
+| `format = "format string {}"` | Formats a field using a format string. Must contain a placeholder (`{}`) with modifiers of your choice. |
+| `with = path::to::formatter` | Formats a field using `path::to::formatter`. The required signature is `fn(&T, &mut std::fmt::Formatter) -> std::fmt::Result` where `T` is a type compatible with the field's type (i.e. the function can be generic and coercions apply). |