[refactor] do some works

- move gitignore in individual directories.
- change some directory layout.
- refactor NlpCodec but not finished.

NlpSrc/README.md

@@ -1,6 +1,12 @@
 # Nlp Src
 
-Useful comparing differences command:
+This directory stores the vanilla NLP files copied from installed Virtools.
+
+This directory may contains some processed temporary files if you are building this repository. These files are ignored by git so don't worry about them.
+
+## Useful Commands
+
+Compare plain text NLP file between different Virtools version when you have decompressed and decrypted vanilla NLP files.
 
 `diff -u --strip-trailing-cr VT25.json VT50.json`
 `diff -u --strip-trailing-cr VT25.txt VT50.txt`