contributing guide added

CONTRIBUTING.md

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+# Contributing to ReVoltVPN
+
+We would love for you to contribute to ReVoltVPN and help make it even better than it is today!
+As a contributor, here are the guidelines we would like you to follow:
+
+## <a name="commit"></a> Commit Message Format
+
+*This specification is inspired by the Angular commit message format.*
+
+#### <a name="commit-header"></a>Commit Message Header
+
+```
+<type>(<scope>): <short summary>
+  │       │             │
+  │       │             └─⫸ Summary in present tense. Not capitalized. No period at the end.
+  │       │
+  │       └─⫸ Commit Scope: app|server|xray|nginx|docs|config
+  │
+  └─⫸ Commit Type: build|ci|docs|feat|fix|perf|refactor|test
+```
+
+The `<type>` and `<summary>` fields are mandatory, the `(<scope>)` field is optional.
+
+##### Type
+
+Must be one of the following:
+
+* **build**: Changes that affect the build system or external dependencies (example: Flutter, pub, Gradle)
+* **ci**: Changes to CI configuration files and scripts
+* **docs**: Documentation only changes (README, AGENTS.md, comments)
+* **feat**: A new feature
+* **fix**: A bug fix
+* **perf**: A code change that improves performance
+* **refactor**: A code change that neither fixes a bug nor adds a feature
+* **test**: Adding missing tests or correcting existing tests
+* **style**: Changes that do not affect the meaning of the code (formatting, whitespace)
+
+##### Scope
+
+The scope should be the part of the project affected:
+
+* `app` — Flutter client (screens, components, logic)
+* `server` — Python Hivemind backend
+* `xray` — Xray Reality config or tunnel behavior
+* `nginx` — nginx routing, SSL, auth
+* `docs` — documentation files
+* `config` — app_config, .gitignore, pubspec
+
+##### Summary
+
+Use the summary field to provide a succinct description of the change:
+
+* use the imperative, present tense: "change" not "changed" nor "changes"
+* don't capitalize the first letter
+* no dot (.) at the end
+
+
+#### <a name="commit-body"></a>Commit Message Body
+
+Just as in the summary, use the imperative, present tense: "fix" not "fixed" nor "fixes".
+
+Explain the motivation for the change in the commit message body. This commit message should explain _why_ you are making the change.
+You can include a comparison of the previous behavior with the new behavior in order to illustrate the impact of the change.
+
+#### Examples
+
+```
+feat(server): add swarm monitoring endpoint
+
+GET /api/swarm returns sorted drone list for the TUI dashboard.
+Persistent total GB saved to swarm_total.json, survives restarts.
+```
+
+```
+fix(app): remove dead _isThrottled field from session timer
+
+Field was written but never read — throttle handled by port-swap now.
+Also removed unreachable 404 branch in _syncWithHivemind.
+```
+
+```
+docs: update README with correct quota and Reality protocol details
+```