docs: improve Readme

Fix #106

* Show the effect of PostCSS Calc alone, so people don't expect it to resolve
  custom properties.
* Remove outdated Travis badge

README.md

@@ -1,12 +1,9 @@
 # PostCSS Calc [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS" width="90" height="90" align="right">][PostCSS]
 
 [![NPM Version][npm-img]][npm-url]
-[![Build Status][cli-img]][cli-url]
 [![Support Chat][git-img]][git-url]
 
-[PostCSS Calc] lets you reduce `calc()` references whenever it's possible. This
-can be particularly useful with the [PostCSS Custom Properties] plugin.
-
+[PostCSS Calc] lets you reduce `calc()` references whenever it's possible.
 When multiple units are mixed together in the same expression, the `calc()`
 statement is left as is, to fallback to the [W3C calc() implementation].
 
@@ -34,61 +31,27 @@ var output = postcss()
   .css
 ```
 
-**Example** (with [PostCSS Custom Properties] enabled as well):
-
-```js
-// dependencies
-var fs = require("fs")
-var postcss = require("postcss")
-var customProperties = require("postcss-custom-properties")
-var calc = require("postcss-calc")
-
-// css to be processed
-var css = fs.readFileSync("input.css", "utf8")
-
-// process css
-var output = postcss()
-  .use(customProperties())
-  .use(calc())
-  .process(css)
-  .css
-```
-
 Using this `input.css`:
 
 ```css
-:root {
-  --main-font-size: 16px;
-}
-
-body {
-  font-size: var(--main-font-size);
-}
-
 h1 {
-  font-size: calc(var(--main-font-size) * 2);
+  font-size: calc(16px * 2);
   height: calc(100px - 2em);
-  margin-bottom: calc(
-      var(--main-font-size)
-      * 1.5
-    )
+  width: calc(2*var(--base-width));
+  margin-bottom: calc(16px * 1.5);
 }
 ```
 
 you will get:
 
 ```css
-body {
-  font-size: 16px
-}
-
 h1 {
   font-size: 32px;
   height: calc(100px - 2em);
+  width: calc(2*var(--base-width));
   margin-bottom: 24px
 }
 ```
-
 Checkout [tests] for more examples.
 
 ### Options
@@ -159,6 +122,9 @@ div[data-size="calc(3*3)"] {
 
 ---
 
+## Related PostCSS plugins
+To replace the value of CSS custom properties at build time, try [PostCSS Custom Properties].
+
 ## Contributing
 
 Work on a branch, install dev-dependencies, respect coding style & run tests
@@ -175,8 +141,6 @@ npm test
 
 ## [License](LICENSE)
 
-[cli-img]: https://img.shields.io/travis/postcss/postcss-calc/master.svg
-[cli-url]: https://travis-ci.org/postcss/postcss-calc
 [git-img]: https://img.shields.io/badge/support-chat-blue.svg
 [git-url]: https://gitter.im/postcss/postcss
 [npm-img]: https://img.shields.io/npm/v/postcss-calc.svg