Add note about web3[tester] in documentation.

[relyt29]

What was wrong?

Related to Issue #1321- Carver pointed out where to add a note about the extras package for tester, so I added it to the RST

How was it fixed?

with vim, and about 20 seconds of my time.

Cute Animal Picture

  _--_                                     _--_
/#()# #\         0             0         /# #()#\
|()##  \#\_       \           /       _/#/  ##()|
|#()##-=###\_      \         /      _/###=-##()#|
 \#()#-=##  #\_     \       /     _/#  ##=-#()#/
  |#()#--==### \_    \     /    _/ ###==--#()#|
  |#()##--=#    #\_   \!!!/   _/#    #=--##()#|
   \#()##---===####\   O|O   /####===---##()#/
    |#()#____==#####\ / Y \ /#####==____#()#|
     \###______######|\/#\/|######______###/
        ()#O#/      ##\_#_/##      \#O#()
       ()#O#(__-===###/ _ \###===-__)#O#()
      ()#O#(   #  ###_(_|_)_###  #   )#O#()
      ()#O(---#__###/ (_|_) \###__#---)O#()
      ()#O#( / / ##/  (_|_)  \## \ \ )#O#()
      ()##O#\_/  #/   (_|_)   \#  \_/#O##()
       \)##OO#\ -)    (_|_)    (- /#OO##(/
        )//##OOO*|    / | \    |*OOO##\\(
        |/_####_/    ( /X\ )    \_####_\|
       /X/ \__/       \___/       \__/ \X\
      (#/                               \#)

[kclowes]

I left a minor grammar nitpick, but looks good otherwise! Thank you!

[relyt29]

Okay, made changes, thanks for the helpful hint on how to render the documentation locally - I think this should be good for another review

[kclowes]

Sorry, I lost track of this. Looks good! Thank you!