Update 07_Selenium_Grid.md

Added Sub-Sections under Why Grid?
Update 06_Worst_Practices.md

adding content for CAPTCHA section
Merge pull request #1 from hoanluu/patch-1

Update 07_Selenium_Grid.md
Update 07_Selenium_Grid.md
Merge pull request #2 from nicegraham/master

CAPTCHA update
Merge pull request #3 from NRezek/patch-1

Update 07_Selenium_Grid.md
Update 06_Worst_Practices.md
Merge pull request #6 from jimevans/master

Updating worst practices section of Chapter 6