Add contributing guidelines, and a page explaining the repo's file and folder structure.

codebase_structure.md

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+# Codebase Structure
+
+These are the folders and some key files used to generate and store various unified hosts files.
+
+## Data folders
+
+There are two facets of the hosts files generated by
+[/updateHostsFile.py](https://github.com/StevenBlack/hosts/blob/master/updateHostsFile.py):
+
+### Basic hosts
+Basic hosts are domains that purvey advertising, malware, tracking, coin
+mining and other abominable behavior. These hosts are included in all our products.
+
+The [/data](https://github.com/StevenBlack/hosts/tree/master/data) folder holds the
+metadata and latest hosts for all our basic hosts sources. The hosts are updated
+automatically by`/updateHostsFile.py` which iterates all the subfolders within `./data`.
+
+### Optional hosts extensions
+Hosts extensions include domains that push potentially addictive content like `porn`, `fake news`,
+`gambling`, and `social` media.
+
+The [/extensions](https://github.com/StevenBlack/hosts/tree/master/extensions) folder holds
+metadata and latest hosts for all our basic hosts sources. The hosts are updated
+automatically by`/updateHostsFile.py` which iterates all the subfolders within `./extensions`.
+
+## Product folders
+
+### Basic hosts
+The [basic hosts file](https://github.com/StevenBlack/hosts/blob/master/hosts), containing
+just adware, malware, etc, is store in the root folder of this repository.
+
+The [/hosts/alternates](https://github.com/StevenBlack/hosts/tree/master/alternates) this folder
+contains the products of `updateHostsFile.py`, one subfolder for each permutation of
+fakenews, gambling, porn, and social media hosts, all of which include the base extensions
+containing hosts that block adware, malware, tracking, etc.
+
+## Other notable files
+
++ [./readme_template.md](https://github.com/StevenBlack/hosts/blob/master/readme_template.md):
+is used to generate the all the various readme files in this repository.  Therefore,
+if you wish to change the readme, you need to change only this file.