Update install doc with instructions for Windows

Failed to install tensorflow-graphics on Windows

OS: Windows 10 Enterprise, 2019, 64 Bit
Reference page: https://www.tensorflow.org/graphics/install
Command used: pip install --upgrade tensorflow-graphics
Errors:

  ERROR: Command errored out with exit status 1:
   command: 'C:\Users\<>\AppData\Local\Continuum\anaconda3\envs\tf2\python.exe' -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\<>\\AppData\\Local\\Temp\\pip-install-7rwn720y\\OpenEXR\\setup.py'"'"'; __file__='"'"'C:\\Users\\<>\\AppData\\Local\\Temp\\pip-install-7rwn720y\\OpenEXR\\setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d 'C:\Users\<>\AppData\Local\Temp\pip-wheel-fo90orx6' --python-tag cp36
       cwd: C:\Users\<>\AppData\Local\Temp\pip-install-7rwn720y\OpenEXR\
			 

Looks like installing OpenEXR is not Optional or for Linux, as specified at the https://www.tensorflow.org/graphics/install

Getting OpenEXR installed via pip install --upgrade OpenEXR fails with following error:

OpenEXR.cpp(36): fatal error C1083: Cannot open include file: 'ImathBox.h': No such file or directory
 error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\BuildTools\\VC\\Tools\\MSVC\\14.23.28105\\bin\\HostX86\\x64\\cl.exe' failed with exit status 2
 ----------------------------------------
 ERROR: Failed building wheel for OpenEXR

Reason: Imathbox.h missing, due to missing libopenexr-dev, need to install as per OS.

Workarounds tried:

• conda install -c conda-forge openexr [installed without error, but then rerun of pip install --upgrade tensorflow-graphics failed again.
• pip install git+https://github.com/jamesbowman/openexrpython.git FAILED
• Install OpenEXR from https://www.lfd.uci.edu/~gohlke/pythonlibs/#openexr (Unofficial) with appropriate python version
  pip install Downloads\OpenEXR-1.3.2-cp36-cp36m-win_amd64.whl for Python 3.6.9, then re-run pip install --upgrade tensorflow-graphics. WORKED!!

Suggestion: Update INSTALL instructions accordingly.

Start with the TensorFlow Graphics install page referenced in the issue and review the instructions for the pip install --upgrade tensorflow-graphics command on Windows. Confirm the documented OpenEXR dependency and Windows workaround, then verify that the installation succeeds with the Python 3.6 environment described in the report.