docs: create CONTRIBUTING.md

The contributing markdown is intended to serve as a short guide for new contributors. Currently it gives a short description about adding a new layer. Other guidelines can be added to it as well.

addresses publiclab#136

Author: rkpattnaik780

CONTRIBUTING.md

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+Contributing to Leaflet Environmental Layers
+==========================
+
+We welcome community contributions to Leaflet Environmental Layers! For local installation, follow the instructions in the [README.md file](./README.md).
+
+We especially welcome contributions from people from groups underrepresented in free and open source software!
+
+Our community aspires to be a respectful place. Please read and abide by our [Code of Conduct](https://publiclab.org/conduct).
+
+## First Timers Welcome!
+
+New to open source/free software? See our WELCOME PAGE, including a selection of issues we've made especially for first-timers. We're here to help, so just ask if one looks interesting:
+
+* https://code.publiclab.org/#r=all
+* https://github.com/publiclab/leaflet-environmental-layers/issues?q=is%3Aopen+is%3Aissue+label%3Afirst-timer-only
+
+Thank you so much!
+
+Learn more about contributing to Public Lab code projects on these pages:
+
+* https://publiclab.org/developers
+* https://publiclab.org/contributing-to-public-lab-software
+* https://publiclab.org/soc
+* https://publiclab.org/wiki/developers
+* https://publiclab.org/wiki/gsoc-ideas
+
+## Adding a new layer
+
+The library consists of multiple layers. It needs to add several other layers. There are issues pertaining to [new layers](https://github.com/publiclab/leaflet-environmental-layers/issues?q=is%3Aopen+is%3Aissue+label%3Anew-layer). 
+
+### Steps to add a new layer :
+1. Find an issue labelled <i>new layer</i>.
+2. Go through the documentaion provided to understand how the API works.
+3. Here's a codepen example that consists of a base layer and [Luftdaten Layer](https://github.com/publiclab/leaflet-environmental-layers/issues/88) as an overlay. You can check its full implementation [here](https://github.com/publiclab/leaflet-environmental-layers/pull/137).
+
+### Some sample PRs to be referred for adding new layers:
+* Osm landfill mine quarry : [#94](https://github.com/publiclab/leaflet-environmental-layers/pull/94)
+* Indigenous Lands Treaties Layer : [#78](https://github.com/publiclab/leaflet-environmental-layers/pull/78)
+
+
+## Bug reports
+
+If you are submitting a bug/issue , please go to https://github.com/publiclab/leaflet-environmental-layers/issues/new