docs: Create CONTRIBUTING.md

Signed-off-by: tison <[email protected]>

CONTRIBUTING.md

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+# Contributing Guide
+
+Thanks for your help in improving the project!
+
+There are two typical contributions to this project:
+
+1. Add support for new languages. You can refer to https://github.com/korandoru/hawkeye/pull/124 as an example.
+2. Add support for new license templates. You can refer to https://github.com/korandoru/hawkeye/pull/148 as an example.
+
+You can find the core concepts with names listed below to understand the design better:
+
+* `DocumentType` defines how a file in a specific type should be handled.
+* `HeaderDef` defines the format of a specific header; for example, `SCRIPT_STYLE` will construct the header format for scripts like `.sh` or `.py` files.
+* `HeaderStyle` is the serde layer for `HeaderDef`.
+* `Selection` describes how to find the files to be handled.
+* `HeaderParser` extracts the header slice from a source file.