Add manual for using towncrier in CONTRIBUTING.md (#351)

* add manual for using towncrier in CONTRIBUTING.md

* Add changelog fragment

* changes after feedback

* Update CONTRIBUTING.md

Co-authored-by: Lukas Heumos <lukas.heumos@posteo.net>

---------

Co-authored-by: Lukas Heumos <lukas.heumos@posteo.net>

Author: seohyonkim

CONTRIBUTING.md

@@ -75,6 +75,57 @@ All chapters are available as Jupyter Notebooks and end-to-end executable.
 The diverse requirements of tools for the chapters do not allow it for us to provide a single environment that can build all chapters.
 Hence, we decided to provide minimal Conda environments per chapter. These can be found in the respective folders.
 
+## Adding changelog entries with `towncrier`
+
+We use `towncrier` to manage our changelog. Here’s how to include a changelog entry when making a PR:
+
+1. Install `towncrier` (only once):
+
+```bash
+pip install towncrier
+```
+
+2. Make your pull request as usual.
+
+3. After opening your PR, note the PR number (e.g., 34), and create a changelog fragment:
+
+```bash
+towncrier create -c 'update blah blah ([#34](https://github.com/theislab/single-cell-best-practices/pull/34)) <sub>@seohyonkim</sub>' 34.changed.md
+```
+
+Replace "update blah blah" with a brief description of your change, PR number with your PR number, and the author of the PR with your github tag.
+Valid categories for the filename of the `markdown` are:
+
+`added`
+`changed`
+`fixed`
+`removed`
+
+4. This will create a `.md` file (e.g., `34.changed.md`) in the `changelog.d/` directory (at the root of the repo). Make sure this file is included in your commit.
+
+5. Push your changes again.
+
+### Releasing a new version (maintainers only)
+
+To release a new version:
+
+1. Run Towncrier to build the changelog:
+
+```bash
+towncrier build --yes --version 2.0.0
+```
+
+This will update `CHANGELOG.md` and remove the `changelog.d/` directory.
+
+2. Add contributor names and links to the PR manually under each relevant PR entry in the generated `CHANGELOG.md`.
+
+3. Recreate the `changelog.d/` directory for future PRs:
+
+```bash
+mkdir changelog.d
+touch changelog.d/.gitkeep
+```
+
 ## Environment setup
 
 Run the following command with the environment file of choice to create the environment for the chapter that you want to build.

changelog.d/345.added.md

@@ -1 +1 @@
-Add new feature `changelog`. Changelog fragments will be added in `changelog` chapter.
+Add new feature `changelog`. Changelog fragments will be added in `changelog` chapter. ([#345](https://github.com/theislab/single-cell-best-practices/pull/345)) <sub>@seohyonkim</sub>

changelog.d/347.changed.md

@@ -1 +1 @@
-Add dataset generator and updatae texts in `Interoperability` chapter
+Add dataset generator and updatae texts in `Interoperability` chapter. ([#347](https://github.com/theislab/single-cell-best-practices/pull/347)) <sub>@seohyonkim</sub>

changelog.d/348.added.md

@@ -1 +1 @@
-Add env- and lamin-setup dropdown with `make dropdown`
+Add env- and lamin-setup dropdown with `make dropdown`. ([#348](https://github.com/theislab/single-cell-best-practices/pull/348)) <sub>@LuisHenzlmeier</sub>

changelog.d/351.added.md

@@ -0,0 +1 @@
+Add manual to use `towncrier` in `CONTRIBUTING.md`. ([#351](https://github.com/theislab/single-cell-best-practices/pull/351)) <sub>@seohyonkim</sub>