improve issue reporting guidelines

Author: yyx990803

.github/CONTRIBUTING.md

@@ -20,14 +20,20 @@ Hi! I’m really excited that you are interested in contributing to Vue.js. Befo
 
 - Check if the issue is reproducible with the latest stable version of Vue. If you are using a pre-release, please indicate the specific version you are using.
 
-- It is **required** that you clearly describe the steps necessary to reproduce the issue you are running into. Issues with no clear repro steps will not be triaged. If an issue labeled "need repro" receives no further input from the issue author for more than 5 days, it will be closed.
+- It is **required** that you clearly describe the steps necessary to reproduce the issue you are running into. Although we would love to help our users as much as possible, diagnosing issues without clear reproduction steps is extremely time-consuming and simply not sustainable.
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+- Use only the minimum amount of code necessary to reproduce the unexpected behavior. A good bug report should isolate specific methods that exhibit unexpected behavior and precisely define how expectations were violated. What did you expect the method or methods to do, and how did the observed behavior differ? The more precisely you isolate the issue, the faster we can investigate.
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+- Issues with no clear repro steps will not be triaged. If an issue labeled "need repro" receives no further input from the issue author for more than 5 days, it will be closed.
 
 - It is recommended that you make a JSFiddle/JSBin/Codepen to demonstrate your issue. You could start with [this template](http://jsfiddle.net/df4Lnuw6/) that already includes the latest version of Vue.
 
 - For bugs that involves build setups, you can create a reproduction repository with steps in the README.
 
 - If your issue is resolved but still open, don’t hesitate to close it. In case you found a solution by yourself, it could be helpful to explain how you fixed it.
 
+- Most importantly, we beg your patience: the team must balance your request against many other responsibilities — fixing other bugs, answering other questions, new features, new documentation, etc. The issue list is not paid support and we cannot make guarantees about how fast your issue can be resolved.
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 ## Pull Request Guidelines
 
 - The `master` branch is basically just a snapshot of the latest stable release. All development should be done in dedicated branches. **Do not submit PRs against the `master` branch.**