Add contributing guide to README

Signed-off-by: Asai Neko <sugar@sne.moe>

README.md

@@ -1,2 +1,25 @@
 # nixcn-cms
 
+## Contribution
+
+1. **Root docs serve the zh-CN version** _[MUST]_
+2. **Use sign-off via `git commit -s`** _[MUST]_
+3. **Do not modify the `main` branch for any reason** _[MUST]_
+4. **Do not omit the commit subject for any reason** _[MUST]_
+5. **Describe all changes in the commit message** _[MUST]_
+6. **Rebase before submitting patches** _[MUST]_
+7. **Commit message written in english** _[MUST]_
+8. **Use OpenPGP/SSH for commit signing** _[MUST]_
+9. **Split commits for large or multi-part changes** _[OPTION]_
+10. **Have fun contributing :)** _[VERY NECESSARY]_
+
+## Toolchain
+
+- Nix
+- Devenv
+- Direnv
+
+## Notice
+
+1. Client and all nix files use 2 space tab.
+2. All Golang files and other configs use 4 space tab.