Fix #575 Updated Sphinx info in docs/README.md

[FranciscoCanas]

• Edited Sphinx installation instructions in README since Sphinx is now in dev-requirements.txt.

[dideler]

Merged as 8b9963cbda00db41ea715c457ac8c29ba9e0eb48, thanks.

I want to point out some amends I made to the commit:

• s/Updated/Update/ in the commit summary
  • Like most projects on git, we try to use the imperative, present tense
• Removed the bullet point from the commit message
  • Bullets are only needed when there are multiple items
• s/Edited/Updated/ in commit message
  • To be consistent with the summary message
  • I should have used "Update", but won't bother fixing that now
• Removed "Fix #575" from the commit summary
  • That issue was already closed, so instead I put "Related to #575" at the bottom of the commit message
• Added [skip ci] so Travis CI skips a build -- a build is not necessary for README changes
• Linked to the dev_requirements.txt file so readers can easily find and inspect the file

These changes were minor enough that I didn't want to delay the PR by suggesting all of them, but keep these small details in mind for writing good commit messages.