Add developer-guide documentation

Signed-off-by: Khosrow Moossavi <khos2ow@gmail.com>

docs/developer-guide/contributing.md

@@ -0,0 +1,12 @@
+---
+title: "Contributing"
+description: "terraform-docs contributing guide."
+menu:
+  docs:
+    parent: "developer-guide"
+weight: 220
+toc: true
+---
+
+Check [CONTRIBUTING.md](https://git.io/JtEzg) file on the root of our repository
+for more details.

docs/developer-guide/plugins.md

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+---
+title: "Plugins"
+description: "terraform-docs plugin development guide."
+menu:
+  docs:
+    parent: "developer-guide"
+weight: 210
+toc: true
+---
+
+If you want to add or change formatter, you need to write plugins. When changing
+plugins, refer to the repository of each plugin and refer to how to build and
+install.
+
+If you want to create a new plugin, please refer to [tfdocs-format-template]. The
+plugin can use [plugin-sdk] to communicate with the host process. You can create a
+new repository from `Use this template`.
+
+[tfdocs-format-template]: https://github.com/terraform-docs/tfdocs-format-template
+[plugin-sdk]: https://github.com/terraform-docs/plugin-sdk