created a .gitignore file

This is almost a copy of `.hgignore`.

**additions to `.hgignore` file**

+. Commented and grouped based on file types.
+. Add `__pycache__` dir and `pyd`, `pyo`

**main difference from .hgignore file**

    # Byte-compiled / optimized / DLL files
    __pycache__/
    *.py[cod]
    
    # Distribution / packaging
    .....<stuffs similar to stuffs in hgignore>....