CONTRIBUTING.md: initial

CONTRIBUTING.md

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+# How to contribute
+
+## Opening issues
+
+* Make sure you have a [GitHub account](https://github.com/signup/free)
+* Submit a ticket for your issue, assuming one does not already exist.
+  * Clearly describe the issue including steps to reproduce when it is a bug.
+
+## Making patches
+
+* Read [Manual (How to write packages for Nix)](https://nixos.org/nixpkgs/manual/)
+* Fork the repository on GitHub.
+* Create a branch for your future fix.
+  * You can make branch from a commit of your local `nixos-version`. That will help you to avoid additional local compilations. Because you will recieve some packages from cache server.
+    * For example: `nixos-version` returns `15.05.git.0998212 (Dingo)`. So you can do:
+
+        ```bash
+        git checkout 0998212
+        git checkout -b 'fix/pkg-name-update'
+        ```
+  * Please avoid working directly on the `master` branch.
+* Make commits of logical units. 
+  * If you removed pkgs, made some major changes etc., write about them in `nixos/doc/manual/release-notes/rl-unstable.xml`.
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Format the commit in a following way:
+
+        ```
+        (pkg-name | service-name): (update from -> to | init at version | refactor | etc)
+
+        Additional information.
+        ```
+  * Examples:
+    * `foo-pkg: init at 2.0.1`
+    * `bar-pkg: update 3.0 -> 3.1.1`
+    * `lala-service: add bazBaz option`
+    * `tata-service: refactor config generation`
+* Test your changes. If you work with 
+  * nixpkgs
+    * update pkg -> 
+      * `nix-env -i pkg-name -f <path to your local nixpkgs folder>`
+    * add pkg -> 
+      * Make sure it's in `pkgs/top-level/all-packages.nix`
+      * `nix-env -i pkg-name -f <path to your local nixpkgs folder>`
+    * _If you don't want to install pkg in you profile_. 
+      * `nix-build -A pkg-attribute-name <path to your local nixpkgs folder>/default.nix` and check results in the folder `result`. It will appear in the same directory where you did `nix-build`.
+    * If you did `nix-env -i pkg-name` you can do `nix-env -e pkg-name` to uninstall it from your system.
+  * nixos and it's modules
+    * You can add new module to you `nixos-configuration file` (usually it's `/etc/nixos/configuration.nix`.  
+    And do `sudo nixos-rebuild test -I nixpkgs=<path to your local nixpkgs folder> --fast`
+* If you have commits `pkg-name: oh, forgot to insert whitespace`: squash commits in this case. Use `git rebase -i`.
+* Rebase you branch against current `master`.
+
+## Submitting changes
+
+* Push your changes to your fork of nixpkgs.
+* Create pull request.
+  * Write the title in format `(pkg-name | service): improvement`
+    * If you update the pkg, write versions `from -> to`
+  * Write in comment if you have tested your patch. Do not rely much on `TravisCL`.
+  * If you make an improvement, write why is it good.
+  * Notify maintainers of the package. For example add to the message: `cc @jagajaga @domenkozar`
+
+## Hotfixing pull requests
+
+* Make the appropriate changes in you branch.
+* Don't create additional commits.
+  * `git rebase`
+  * `git push --force` to your branch.