Add issue template

Add an issue template to encourage Homebrew/install issues to be
better filled out.

.github/ISSUE_TEMPLATE.md

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+# Please fill out one of the templates on: https://github.com/Homebrew/install/issues/new/choose or we will close it without comment.

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: New issue for Reproducible Bug
+about: "If you're sure it's reproducible and not just your machine: submit an issue so we can investigate."
+
+---
+
+# Please note we will close your issue without comment if you delete, do not read or do not fill out the issue checklist below and provide ALL the requested information. If you repeatedly fail to use the issue template, we will block you from ever submitting issues to Homebrew again.
+
+- [ ] your problem was from running the official `install` or `uninstall` script?
+- [ ] after installation: ran `brew config` and `brew doctor` and included their output with your issue? If you couldn't install: provided your OS version with the output of your issue?
+
+<!-- To help us debug your issue, please complete these sections: -->
+
+## What you were trying to do (and why)
+
+<!-- replace me -->
+
+## What happened (include command output)
+
+<!-- replace me -->
+
+<details>
+  <summary>Command output</summary>
+  <pre>
+
+  <!-- replace this with the command output -->
+
+  </pre>
+</details>
+
+## What you expected to happen
+
+<!-- replace me -->
+
+## Step-by-step reproduction instructions (by running `brew` commands)
+
+<!-- replace me -->

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+
+contact_links:
+- name: Get help on Discourse
+  url: https://discourse.brew.sh
+  about: Have a question? Not sure if your issue affects everyone reproducibly? The quickest way to get help is on Homebrew's Discourse!
+- name: New issue on Homebrew/brew
+  url: https://github.com/Homebrew/brew/issues/new/choose
+  about: Having a `brew` problem that's not from a `brew install` or `brew upgrade` of a single formula/package?
+- name: New issue on Homebrew/homebrew-core
+  url: https://github.com/Homebrew/homebrew-core/issues/new/choose
+  about: On macOS/Mac OS X? Having a `brew` problem with a `brew install` or `brew upgrade` of a single formula/package? Report it to Homebrew/homebrew-core (the macOS core tap/repository).
+- name: New issue on Homebrew/homebrew-cask
+  url: https://github.com/Homebrew/homebrew-cask/issues/new/choose
+  about: Having a `brew cask` problem? Report it to Homebrew/homebrew-cask (the cask tap/repository).
+- name: New issue on Homebrew/linuxbrew-core
+  url: https://github.com/Homebrew/linuxbrew-core/issues/new/choose
+  about: On Linux? Having a `brew` problem with a `brew install` or `brew upgrade` of a single formula/package? Report it to Homebrew/linuxbrew-core (the Linux core tap/repository).