Fixed some spelling, typos, and grammar issues on the readme.

[jethrolarson]

Sorry about the duplicate pull request. I'm still learning the best way to do pr revisions here.

[davidchambers]

This is the way I update a pull request:

$ # make some changes
$ git add <file> <file>
$ git commit --amend
$ git push origin <branch> --force

These are the commands I run to resolve conflicts in an existing pull request:

$ git checkout master
$ git pull upstream master
$ git checkout -  # "-" is short for "most recent branch"
$ git rebase master
$ # resolve merge conflicts
$ git add <file> <file>
$ git rebase --continue
$ git push origin <branch> --force

[paldepind]

Nice. Thank you!

Also thanks to David. For some odd reason I've gotten the habit of making a new commit and squashing when updating PRs. Using ammend is clearly easier.