Update contributing guide (#12)

CONTRIBUTING.md

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+# Contribution guide
+
+## Automatic updates
+
+Each working cask will be updated weekly automatically by running `auto_updater.sh` through a 
+CircleCi workflow schedule.
+
+The script will enumerate over each cask, search through the releases in their respective 
+`releases.json` and author a pull request if there is a newer version released.
+
+All the releases and their notes are published [here](https://github.com/dotnet/core/tree/master/release-notes).
+
+## Adding a new cask
+
+Install the necessary tools if you already haven't
+
+```
+./brew_install_necessary.sh
+```
+
+Use the existing casks as a template and fill in the version number till the patch major version. The filename and cask name must match, according to BrewCask's rules.
+
+The runtime version, `sha256`, and the `url` isn't important and can be filled with placeholder text.
+
+```
+cask 'dotnet-sdk-2.99.400' do
+  version '2.99.400,2.0.0'
+  sha256 '?????'
+
+  url '???'
+  
+
+  ... so on a so forth ...
+
+end
+```
+
+Then run `auto_updater.sh` in dry run mode.
+
+```
+# passing in no arguments defaults to dry run mode
+./auto_updater.sh
+```
+
+The dry run mode will update all casks, including the cask you've just added without committing and publishing pull
+requests.
+
+Branch out, add the file you want to commit, commit, and push.
+
+```
+git checkout -b new-cask/dotnet-sdk-2.99.400
+git add Casks/dotnet-sdk-2.99.400.rb
+git commit -m "Add support for dotnet-sdk-2.99.400.rb"
+git push fork new-cask/dotnet-sdk-2.99.400
+```
+
+