Merge pull request mozilla#11542 from Snuffleupagus/dist-lib-readme

Add a README file to the `/lib` folder in `pdfjs-dist` (issue 11539)

Author: timvandermeij

external/dist/lib/README.md

@@ -0,0 +1,7 @@
+It's *very highly* recommended to *not* use these files, but rather use the
+pre-built library as found in e.g. the `/build`, `/web`, and `/image_decoders`
+folders in the root of this repository.
+
+Please note that the "lib" build target exists mostly to enable unit-testing in
+Node.js/Travis, and that you'll need to handle e.g. any Node.js dependencies
+yourself if using the files in this folder.