Add a basic contributing guide

Copied from https://github.com/fatiando/data with a note
to make some changes to adding new datasets.

Author: leouieda

CONTRIBUTING.md

@@ -0,0 +1,98 @@
+# Contributing Guidelines
+
+:tada: **First off, thank you for considering contributing to our project!** :tada:
+
+This is a community-driven project, so it's people like you that make it useful and
+successful.
+
+If you get stuck at any point you can create an issue on GitHub (look for the *Issues*
+tab in the repository) or contact us at one of the other channels mentioned below.
+
+
+## General Guidelines
+
+For general information about contributing to open-source and the Fatiando a Terra
+projects, please refer to our [standard Contributing Guide][contrib].
+
+This document also contains guidelines specific to this repository below.
+
+
+## Ground Rules
+
+The goal is to maintain a diverse community that's pleasant for everyone.
+**Please be considerate and respectful of others**.
+Everyone must abide by our [Code of Conduct][coc] and we encourage all to read 
+it carefully.
+
+
+## Requirements for datasets
+
+The following are the requirements that datasets need to meet in oder to be 
+considered for this project.
+
+> **Definitions:**
+> 
+> * *Source dataset*: the original data as distributed by the data owners/creators.
+> * *Output dataset*: the modified/repackaged version that we distribute.
+> * *FAIR data*: data that follows the [FAIR principles][fair].
+
+Source datasets must:
+
+1. Be FAIR data: either in the public domain or distributed under an open licence that does not
+   place restrictions on reuse beyond attribution or using the same license. 
+   For example, CC-BY and CC-BY-SA are acceptable but not CC-BY-NC.
+1. Represent a common real-world application.
+1. Contain interesting features that **lead to teachable moments** for tutorials.
+   for example, interesting anomalies easily associated with geology, large gaps in
+   bathymetry lead to interesting interpolation issues, etc.
+
+Output datasets should:
+
+1. Contain standard and descriptive variable names. For example, "longitude" 
+   instead of "LON", "gravity_disturbance_mgal" instead of "FAA", "easting_m"
+   instead of "x". 
+1. Include associated metadata (datum, license, source, etc.) if supported
+   by the format. For example, netCDF metadata following CF conventions
+   through `.attrs` attributes in xarray.
+3. Specify units through appropriate metadata (CF conventions in netCDF or
+   column names in CSV, like `gravity_disturbance_mgal`). Exceptions are
+   longitude and latitude coordinates which are always in decimal degrees.
+1. Strive to be under 10 Mb in size, if possible. This keeps downloads fast,
+   particularly when building documentation and testing on CI. Use compression 
+   when appropriate and only if it doesn't add difficult to install dependencies.
+   Larger files may be considered but should not be used in code that runs on 
+   CI to avoid long build times and overloading the data servers.
+   
+   
+## Adding a new dataset
+
+1. **Propose a new dataset:** First, open an Issue in [][issue] with information about the 
+   proposed dataset for discussion.
+
+THE FOLLOWING NEEDS TO BE UPDATED.
+
+Follow these guidelines to prepare the dataset:
+
+* See our [standard Contributing Guide][contrib] for instructions on creating 
+  pull requests and setting up your environment.
+* Create a folder following the naming convention `location_datatype` (all lower
+  case and separated by `_`).
+* Inside that folder, create a Jupyter notebook called `prepare.ipynb` with the
+  code for downloading (using [Pooch](https://github.com/fatiando/pooch)),
+  formatting (cleaning, slicing, datum conversion, etc), and exporting the 
+  new dataset. Follow the conventions in the other notebooks.
+* If any new dependencies are required to prepare the dataset, add them to the
+  `environment.yml` file.
+* The output dataset should follow the same naming convention as the folder:
+  `location_datatype.extension`.
+* The notebook should create a `preview.jpg` image with a plot of the output
+  dataset for easy inspection.
+* If the original data can't be automatically downloaded in the notebook and it
+  is under 50 Mb, you may include it in the repository. Feel free to use 
+  compression to reduce the size of the file(s).
+* Include the information about the new dataset in the `README.md` file.
+
+
+[contrib]: https://github.com/fatiando/community/blob/main/CONTRIBUTING.md
+[coc]: https://github.com/fatiando/community/blob/main/CODE_OF_CONDUCT.md
+[fair]: https://www.go-fair.org/fair-principles/