Chore: Added docs for the new attribute.

Author: StringEpsilon

docs/Attributes/PhpSerializationFilter.md

@@ -0,0 +1,54 @@
+
+[back to overview](../README.md)
+
+---
+
+**Table of contents**
+- [PhpSerializationFilter](#phpserializationfilter)
+	- [Example](#example)
+
+---
+
+# PhpSerializationFilter
+
+Abstract class that can be implemented to modifiy the serialization output on a given property or field.
+To facilitate completely omitting fields, `null` returned from the `Serialize` method has special consideration in the
+serializer.
+
+## Example
+
+**Implementation:**
+```cs
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
+public class PhpIgnoreNull : PhpSerializationFilter {
+	public override string? Serialize(object key, object value, PhpSerializiationOptions options) {
+		if (value != null) {
+			return PhpSerialization.Serialize(key, options) + PhpSerialization.Serialize(value, options);
+		}
+		return null;
+	}
+}
+```
+
+**Usage:**
+```cs
+public struct PersonA {
+	[PhpIgnoreNull]
+	public string? Title { get; set; }
+	public required string FirstName { get; set; } 
+	public required string LastName { get; set; } 
+}
+
+var serialized = PhpSerialization.Serialize(new PersonA { FirstName = "John", LastName = "Johnson" });
+// serialized = "a:2:{s:9:"FirstName";s:4:"John";s:8:"LastName";s:7:"Johnson"}"
+
+// in contrast without the custom filter:
+public struct PersonB {
+	public string? Title { get; set; }
+	public required string FirstName { get; set; } 
+	public required string LastName { get; set; } 
+}
+
+var serialized = PhpSerialization.Serialize(new PersonB { FirstName = "John", LastName = "Johnson" });
+// serialized = "a:3:{s:5:"Title";N;s:9:"FirstName";s:4:"John";s:8:"LastName";s:7:"Johnson"}"
+```

docs/README.md

@@ -14,6 +14,7 @@ You can make some decisions about de/serialization directly on your class and st
 * [PhpClass](./Attributes/PhpClass.md)
 * [PhpIgnore](./Attributes/PhpIgnore.md)
 * [PhpProperty](./Attributes/PhpProperty.md)
+* [PhpSerializationFilter](./Attributes/PhpSerializationFilter.md)
 
 ### Options