Update contributing guide

Author: uriyyo

docs/contributing.md

@@ -5,3 +5,118 @@ Any and all contributions and involvement with the project is welcome. The easie
 ## Documentation
 
 The documentation is built using [mkdocs](https://www.mkdocs.org/). All documentation is in markdown format, and can be found in `./docs/`
+
+
+## Contributing Code
+
+### Step 1: prerequisites
+
+`fastapi-pagination` uses [poetry](https://python-poetry.org/) for dependency management.
+Please, install poetry before continuing.
+
+Minimum supported python version is `3.8`.
+
+
+### Step 2: clone the repo
+
+```shell
+git clone https://github.com/uriyyo/fastapi-pagination
+```
+
+
+### Step 3: install dependencies
+
+To install all dependencies, run:
+```sh
+poetry install -E all
+```
+
+To install only basic dependencies, run:
+```sh
+poetry install
+```
+
+To install docs requirements, run:
+```sh
+poetry run pip install -r docs_requirements.txt
+```
+
+### Step 4: do your changes
+
+If you want to add new feature, please, create an issue first and describe your idea.
+
+If you want to add new extension for pagination you will need to create a new module in `fastapi_pagination/ext/` directory.
+Please, use `fastapi_pagination/ext/sqlalchemy.py` as an example.
+Generally, you will need to call function `paginate` and signature should include next arguments:
+```py
+from typing import Any, Optional
+
+from fastapi_pagination.api import apply_items_transformer, create_page
+from fastapi_pagination.bases import AbstractParams
+from fastapi_pagination.types import AdditionalData, AsyncItemsTransformer
+from fastapi_pagination.utils import verify_params
+
+
+async def paginate(
+    query: QuerySet,
+    params: Optional[AbstractParams] = None,  # Optional params for pagination (if None, current params will be used)
+    *,
+    transformer: Optional[AsyncItemsTransformer] = None,  # Optional transformer for items
+    additional_data: Optional[AdditionalData] = None,  # Optional additional data for page object
+) -> Any:
+    params, raw_params = verify_params(params, "limit-offset")  # verify params is of correct type
+
+    total = await query.count()  # get total count of items
+    items = await query.limit(raw_params.limit).offset(raw_params.offset).all()  # get items for current page
+    t_items = await apply_items_transformer(items, transformer, async_=True)  # apply transformer if needed
+
+    return create_page(  # create page object
+        t_items,
+        total=total,
+        params=params,
+        **(additional_data or {}),
+    )
+```
+
+If you want to add/updated docs, then you will need to edit `./docs/` directory.
+You can run `mkdocs serve` to see your changes locally.
+
+### Step 5: run pre-commit hooks
+
+Before creating a commit, please, run pre-commit hooks:
+```sh
+poetry run pre-commit run --all-files
+```
+
+You can also install pre-commit hooks to run automatically before each commit:
+```sh
+poetry run pre-commit install
+```
+
+### Step 6: run tests
+
+To run tests, run:
+```sh
+poetry run pytest tests
+```
+
+If you want to run tests with coverage, run:
+```sh
+poetry run pytest tests --cov=fastapi_pagination
+```
+
+If you want to run only unit tests, run:
+```sh
+poetry run pytest tests --unit-tests
+```
+
+If you want to run only integration tests, then you will also will need to have `PostgreSQL`, `MongoDB` and `Casandra` running.
+
+To run integration tests, run:
+```sh
+poetry run pytest tests/ext
+```
+
+### Step 7: create a pull request
+
+After you have done all changes, please, create a pull request.