Description of External library imports into Moodle

Flexitour Instructions
----------------------
1. Clone https://github.com/andrewnicols/flexitour into an unrelated directory
2. Copy /build/tour.js to amd/src/tour.js
3. Open the amd/src/tour.js file and find the AMD module define.
4. Change the "popper" inclusion to "core/popper"
5. Update thirdpartylibs.xml
6. Run `grunt amd`
