Add contribution section

CONTRIBUTING.md

@@ -87,6 +87,11 @@ functionality must be part of a major version release.
 See [Jackson Releases on the wiki](https://github.com/FasterXML/jackson/wiki/Jackson-Releases)
 for more information.
 
+#### Coding Style
+
+Jackson projects usually adhere to well-known conventional coding guides, but there still have
+Jackson-specific coding conventions. Please refer to [Jackson Code Style](../../blob/master/contribution/jackson-java-coding-style-guide.md) for details.
+
 #### Testing
 
 Jackson's functionality is vast and is used widely, so automated testing for any changes is

contribution/jackson-java-coding-style-guide.md

@@ -0,0 +1,50 @@
+# Jackson-Java Coding Style Guide
+
+This document serves as the coding conventions for Java source code in the Jackson project. It's inspired by the Google
+Java Style Guide.
+
+## Table of Contents
+
+1. [Source Formatting](#source-formatting)
+    1. [Indentation](#indentation)
+    2. [Import Statements](#import-statements)
+        1. [Wildcard Imports](#wildcard-imports)
+        2. [Ordering](#ordering)
+2. [Naming Conventions](#naming-conventions)
+    1. [Field Naming](#field-naming)
+    2. [Method Naming](#method-naming)
+
+## Source Formatting
+
+### Indentation
+
+- **Rule**: Use spaces instead of tabs.
+- **Amount**: 4 spaces per indentation level.
+
+### Import Statements
+
+#### Wildcard Imports
+
+- **Usage**: Wildcards are permissible.
+- **Threshold**: Employ wildcards for 5 or more imports from the same package.
+- **Exceptions**: For frequently used packages like `java.util`, `java.io`, and main Jackson packages (
+  e.g., `com.fasterxml.jackson.databind`), wildcards may be used for fewer imports.
+
+#### Ordering
+
+1. **JDK Imports**: Begin with standard Java (JDK) imports.
+2. **Jackson Imports**: Follow with Jackson imports, starting from base types (annotations, core, databind), ordered
+   from foundational to higher-level.
+
+## Naming Conventions
+
+### Field Naming
+
+- **Non-Public Fields**: Prefix non-public member fields with an underscore (e.g., `_fieldName`).
+
+### Method Naming
+
+- **Public Methods**: Adhere to standard Java naming conventions.
+- **Internal Methods**: Prefix methods used within the class with an underscore.
+- **Sub-Class Methods**: Optionally, use a leading underscore for methods intended for subclass use, but not for
+  external class calls.