Update JSPM+SystemJS recipe per @jamestalmage.

See istanbuljs/nyc#305 - specifically [this comment and the one after it](istanbuljs/nyc#305 (comment)).

Essentially, @jamestalmage brought up that it would be good to explicitly mention in the recipe that you should not pre-build your project with JSPM/SystemJS to run your tests. The purpose of ava-jspm-loader is that you do not have to build before testing.

Closes #954

Author: fluffywaffles

docs/recipes/jspm-systemjs.md

@@ -2,7 +2,7 @@
 
 Translations: [Français](https://github.com/avajs/ava-docs/blob/master/fr_FR/docs/recipes/jspm-systemjs.md)
 
-It requires a special loader helper to correctly resolve `import`s of JSPM packages when using AVA.
+It requires a special loader helper to correctly resolve `import`s of JSPM packages when using AVA. The purpose of the loader is to allow you to run your tests without having to pre-build your JSPM project.
 
 ## Setup