Add more description on unspecified directory in javascript readme (release notes: no) (#6393)

* Add more description on unspecified directory

In the example code, the "js" folder appeared for the first time and there is no clue where the current folder is. Therefore, there should be some comments describing what the folder is. Otherwise, like me, spent a lot of time guessing what codes - some release version or the source code - should be downloaded to go to the "js" directory.

* Add detailed description on how to run js tests

The test files are directly in the Protocol Buffer project js folder. Add more detailed instruction on how and where to run the tests.

* Correct two spelling mistakes

The two spelling mistakes mentioned in your comments are corrected.