Gotcha/styled jsx and jest (#35)

* docs(jest): add gotcha reg mocking the styled-jsx css module

* docs(jest): fix typos

* docs(jest): rephrased a sentence for extra clarity based on PR comment

gotchas/styled-jsx-and-jest.md

@@ -0,0 +1,63 @@
+# TL;DR (recommended setup)
+
+### Usage of `styled-jsx` `css` and `css.resolve` function
+
+In your code-base, make sure to import the `resolve` function from the named export, like so:
+
+```javascript
+import { resolve } from 'styled-jsx/css';
+const buttonStyle = resolve`button { color: green; }`;
+```
+
+and not as property of the `css` function, i.e. **do not** do this:
+
+```javascript
+import css from 'styled-jsx/css';
+const buttonStyle = css.resolve`button { color: green; }`;
+```
+
+### Mocking the `css` and `css.resolve` function
+
+Create a mock file in the project root, adjacent to the `node_modules` folder: `./__mocks__/styled-jsx/css.js`
+
+```javascript
+module.exports.__esModule = true;
+module.exports.default = () => 'default';
+module.exports.resolve = () => 'resolve';
+module.exports.global = () => 'global';
+```
+
+### Babel config
+
+For the test env, include the `styled-jsx/babel-test` plugin, and make sure to exclude the `styled-jsx/babel` plugin:
+
+```json
+{
+    "presets": ["react-app"],
+    "env": {
+        "production": {
+            "plugins": ["styled-jsx/babel"]
+        },
+        "development": {
+            "plugins": ["styled-jsx/babel"]
+        },
+        "test": {
+            "plugins": ["styled-jsx/babel-test"]
+        }
+    }
+}
+```
+
+# Background
+
+The above setup is required to prevent a problem that arises when using calls to `css.resolve` in your code-base and then trying to mock the `css` function. In its original implementation in `styled-jsx`, `css` is a function, and to this function a property is added, a bit like this:
+
+```javascript
+function css() {}
+css.resolve = function() {};
+module.exports = css;
+```
+
+Now, this is quite irregular, but perfectly valid JavaScript, so it would be very logical to think that the mocked version could follow the same pattern. **However**, `jest` doesn't seem to have support for these type of functions. We haven't been able to get to the bottom of it, but we do know that trying to mock the function as above will just lead to `Cannot read property "resolve" of undefined` errors in the tests.
+
+By always calling the `resolve` function directly, and mocking the `css` function as in the recommended setup section, the above problem is circumvented.