Re-organize repo structure

[khiga8]

As we add more files, I think it'll be beneficial to have a directory structure to place the rules and docs. Maybe something like:

src
--rules
----no-default-alt-text.js
----no-generic-link-text.js
----index.js
--helpers
test
--utils
------no-default-alt-text.test.js
------no-generic-link-text.test.js
docs
--rules
package.json
index.js
README.md