Tweak issue template and contributing guidelines.

- Note issues may be closed without comment if sufficient information is
  not provided.
- Deprecate Homebrew/brew-evolution, note new feature request flow
- Note some issues should be created on Homebrew/brew

.github/ISSUE_TEMPLATE.md

@@ -10,8 +10,8 @@ _You can erase any parts of this template not applicable to your Issue._
 
 ### Bug reports:
 
-Please replace this line with a brief summary of your issue **AND** the output of `brew config` and `brew doctor`.
+Please replace this section with a brief summary of your issue **AND** the output of `brew config` and `brew doctor`. Please note we may immediately close your issue without comment if you do not fill out the issue template and provide the requested information.
 
 ### Propose a feature:
 
-Instead of creating an issue here, please create a pull request with your change proposal in the [Homebrew Evolution](https://github.com/Homebrew/brew-evolution) repository using the [proposal template](https://github.com/Homebrew/brew-evolution/blob/master/proposal_template.md).
+Please replace this section with a detailed description of your proposed feature, the motivation for it and alternatives considered. Please note we may close this issue or ask you to create a pull-request if it's something we're not actively planning to work on.

CONTRIBUTING.md

@@ -6,10 +6,10 @@ First time contributing to Homebrew? Read our [Code of Conduct](https://github.c
 * run `brew update` (twice)
 * run and read `brew doctor`
 * read [the Troubleshooting Checklist](https://github.com/Homebrew/brew/blob/master/docs/Troubleshooting.md#troubleshooting)
-* open an issue on the formula's repository
+* open an issue on the formula's repository or on Homebrew/brew if it's not a formula-specific issue
 
 ### Propose a feature
 
-* Please create a pull request with your change proposal in the [Homebrew Evolution](https://github.com/Homebrew/brew-evolution) repository using the [proposal template](https://github.com/Homebrew/brew-evolution/blob/master/proposal_template.md).
+* open an issue with a detailed description of your proposed feature, the motivation for it and alternatives considered. Please note we may close this issue or ask you to create a pull-request if this is not something we see as sufficiently high priority.
 
 Thanks!