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.

Author: Chen WANG

README.md

@@ -39,9 +39,8 @@ If you want, you can compile `protoc` from source instead.  To do this
 follow the instructions in [the top-level
 README](https://github.com/protocolbuffers/protobuf/blob/master/src/README.md).
 
-Once you have `protoc` compiled, you can run the tests by typing:
+Once you have `protoc` compiled, you can run the tests provided along with our project to examine whether it can run successfully. In order to do this, you should download the Protocol Buffer source code from the release page with the link above. Then extract the source code and navigate to the folder named `js` containing a `package.json` file and a series of test files. In this folder, you can run the commands below to run the tests automatically.
 
-    $ cd js
     $ npm install
     $ npm test