Fix Spelling and Writing Mistakes in the README

[gkapfham]

There are some small mistakes in the writing in the README file.
Here are some examples of mistakes that could be fixed:

• On line 113, "built-in" is not spelled correctly
• On line 172, "additional" is not spelled correctly
• On line 298, "command" is not spelled correctly
• We should decide whether or not we are going to place the $ symbol before
  commands. For instance, line 142 does contain this symbol but line 176 does not.
• A person may not be using pipenv to install this tool or the dependencies
  of the program that they are developing. As such, we might want to make it clear
  that they don't have to write pipenv install <library-name> on line 106. An
  easy way to fix this issue would be to mention that, if the person uses Pipenv
  then they would type this command. Otherwise, they would type the installation
  command that it appropriate for their chosen installation method.