docs: Include Tip about computed env vars (vuejs#1780)

Ray Foss · Akryum · commit 62e28680077a · 2018-07-11T01:28:04.000+02:00
* Include tip about computed env vars

I'm personally using this frivolously in my own footer
```
// vue.config.js
const revision = require('child_process').execSync('git rev-parse HEAD').toString().trim()
const pjson = require('./package.json')

process.env.VUE_APP_COMMIT = `"${revision}"`
process.env.VUE_APP_VERSION = `"${pjson.version}"`
module.exports = {}
```

```
&lt;template&gt;
  &lt;footer&gt;
      &lt;span&gt;v{{ version }} ({{ commit }})&lt;/span&gt;
  &lt;/footer&gt;
&lt;/template&gt;

&lt;script&gt;
export default {
  data: function () {
    return {
      version: process.env.VUE_APP_VERSION,
      commit: process.env.VUE_APP_COMMIT
    }
  }
}
```

# Validation
 - I'm using this.
 - The inline require doesn't cause lint issues on vue-cli v3 config files. I did a full builds to check.

* tip format match

* tip format

* tip, format

* markdown formatting conflicts with es6 templates

Also, quotes, optional now. interesting. I tried it.

* remove template stray

* filenames are not proper english grammar, yet
diff --git a/docs/guide/mode-and-env.md b/docs/guide/mode-and-env.md
@@ -74,6 +74,10 @@ In addition to `VUE_APP_*` variables, there are also two special variables that
 
 All resolved env variables will be available inside `public/index.html` as discussed in [HTML - Interpolation](./html-and-static-assets.md#interpolation).
 
+    ::: tip
+    You can have computed env vars in your `vue.config.js` file. They still need to be prefixed with `VUE_APP_`. This is useful for version info `process.env.VUE_APP_VERSION = require('./package.json').version`
+    :::
+
 ## Local Only Variables
 
 Sometimes you might have env variables that should not be committed into the codebase, especially if your project is hosted in a public repository. In that case you should use an `.env.local` file instead. Local env files are ignored in `.gitignore` by default.
