Create CONTRIBUTING.md -- document procedure (#45)

* Create CONTRIBUTING.md

* update README

Author: jGaboardi

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Procedure
+
+1. *After* opening an issue and discussing with the development team, create a PR with the proposed changes. 
+2. If [testing fails](https://github.com/uscuni/sgeop/actions/runs/11368511561) due to an update in the code base:
+3. Observed data is [saved as artifacts](https://github.com/uscuni/sgeop/actions/runs/11368511561#artifacts) from the workflow and can be download locally.
+4. We determine the `ci_artifacts-ubuntu-latest-py312_sgeop-latest` data as the "truth."
+5. After comparison of the current "known" data with new data from (3.), if new data is "truthier," update your PR with the new "known" data.
+
+## Code Structure
+
+Code should be linted and formatted via `ruff`. With the [`.pre-commit` hooks](https://github.com/uscuni/sgeop/blob/main/.pre-commit-config.yaml) we have code in commits will be formatted and linted automatically once [`pre-commit` is installed](https://pre-commit.com/#installation).

README.md

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 [![Continuous Integration](https://github.com/uscuni/sgeop/actions/workflows/testing.yml/badge.svg)](https://github.com/uscuni/sgeop/actions/workflows/testing.yml) [![codecov](https://codecov.io/gh/uscuni/sgeop/graph/badge.svg?token=GFISMU0WPS)](https://codecov.io/gh/uscuni/sgeop)
 
-Street geometry processing toolkit
+## Introduction
+
+## Documentation
+
+## Examples
 
 ```py
 import sgeop
 
 simplified = sgeop.simplify_network(gdf)
 ```
 
-This package developed & and maintained by:
-* [Martin Fleischmann](https://github.com/martinfleis)
-* [Anastassia Vybornova](https://github.com/anastassiavybornova)
-* [James D. Gaboardi](https://github.com/jGaboardi)
-
-## Introduction
-
-## Documentation
-
-## Examples
-
 ## Contribution
 
+See our guidelines in [`CONTRIBUTING.md`](https://github.com/uscuni/sgeop/CONTRIBUTING.md).
+
 ## Recommended Citation
 
 ---------------------------------------
 
+This package developed & and maintained by:
+* [Martin Fleischmann](https://github.com/martinfleis)
+* [Anastassia Vybornova](https://github.com/anastassiavybornova)
+* [James D. Gaboardi](https://github.com/jGaboardi)
+
 Copyright (c) 2024-, sgeop Developers