README: Associate Apache 2.0 License

From the python.org wiki, the answer to the question [1]:

  "What if I want to contribute my code to the PSF?"

mentions that if code is going to end up in Python or the standard library,
the PSF will require you to license code under "Academic Free License" or
"Apache License 2.0" .

[1] https://wiki.python.org/moin/PythonSoftwareFoundationLicenseFaq#What_if_I_want_to_contribute_my_code_to_the_PSF.3F
README: Update curl command to follow redirect
README: Add instruction to check MD5 sum of the downloaded source package
README: Add instruction to extract Python source archive

Also add mkdir '-p' option to avoid complaining if directories are
already created.
README: Reference license file using relative link