docs: Explain why this project exists

This explanation comes from response to a user question in kentcdodds#4 asking what this was for.

README.md

@@ -29,14 +29,43 @@
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-## The problem
+babel-macros defines a standard interface for libraries that want to use compile-time code transformation without requiring the user to add a babel plugin to their build system (other than babel-macros, which is ideally already in place).
+
+For instance, many css-in-js libraries have a css tagged template string function:
+```js
+const styles = css`
+  .red {
+    color: red;
+  }
+`;
+```
 
-// TODO
+The function compiles your css into (for example) an object with generated class names for each of the classes you defined in your css:
+```js
+console.log(styles); // { red: "1f-d34j8rn43y587t" }
+```
+
+This class name can be generated at runtime (in the browser), but this has some disadvantages:
+* There is cpu usage/time overhead; the client needs to run the code to generate these classes every time the page loads
+* There is code bundle size overhead; the client needs to receive a CSS parser in order to generate these class names, and shipping this makes the amount of js the client needs to parse larger.
+
+To help solve those issues, many css-in-js libraries write their own babel plugin that generates the class names at compile-time instead of runtime:
+```js
+// Before running through babel:
+const styles = css`
+  .red {
+    color: red;
+  }
+`;
+// After running through babel, with the library-specific plugin:
+const styles = { red: "1f-d34j8rn43y587t" };
+```
 
-## This solution
+If the css-in-js library supported babel-macros instead, then they wouldn't need their own babel plugin to compile these out; they could instead rely on babel-macros to do it for them. So if a user already had babel-macros installed and configured with babel, then they wouldn't need to change their babel configuration to get the compile-time benefits of the library. This would be most useful if the boilerplate they were using came with babel-macros out of the box, which is what we're hoping will be true for create-react-app in the future. 
 
-This is a work in progress and the form of the solution will likely change a lot
-before `1.0.0` is released. Collaboration is welcome!
+Although css-in-js is the most common example, there are lots of other things you could use babel-macros for, like:
+* Compiling GraphQL fragments into objects so that the client doesn't need a GraphQL parser
+* Eval-ing out code at compile time that will be baked into the runtime code, for instance to get a list of directories in the filesystem (see [preval](https://github.com/kentcdodds/babel-plugin-preval))
 
 ## Installation