Minor tweaks

Signed-off-by: Mike Allanson <michael.allanson@gmail.com>

CONTRIBUTING.md

@@ -54,8 +54,8 @@ The usual contributing steps are:
   see the [gatsby-dev-cli README](/packages/gatsby-dev-cli/)
 * Add tests and code for your changes.
 * Once you're done, make sure all tests still pass: `yarn test`
-* Commit with ["Signed-off-by"](#developer-certificate-of-origin) line appended
-  to commit message and push to your fork.
+* Commit with a ["Signed-off-by"](#developer-certificate-of-origin) line appended
+  to your commit message and push to your fork.
 * Create a pull request from your branch.
 
 ### Contributing to the documentation.
@@ -79,13 +79,13 @@ a pull request.
 * The Markdown files for the documentation live in `/docs` folder. Make
   additions or modifications here.
 * Make sure to double check your grammar and capitalise correctly.
-* Commit with ["Signed-off-by"](#developer-certificate-of-origin) line appended
-  to commit message and push to your fork.
+* Commit with a ["Signed-off-by"](#developer-certificate-of-origin) line appended
+  to your commit message and push to your fork.
 * Create a pull request from your branch.
 
 ### Developer Certificate of Origin
 
-Gatsby repository enforces the 
+The Gatsby repository enforces the 
 [Developer Certificate of Origin](https://developercertificate.org/) (DCO) on 
 Pull Requests. It requires all commit messages to contain the `Signed-off-by` 
 line with an email address that matches the commit author.
@@ -97,14 +97,14 @@ automatically to your commit message:
 $ git commit -s -m 'This is my commit message'
 ```
 
-If you already created Pull Request and noticed that DCO check is failing, you
-can use this command to append signoff to your commits:
+If you've already created a Pull Request and notice that the DCO check is failing, you
+can use this command to append a signoff message to your commits:
 
 ```shell
 $ git rebase --signoff
 ```
 
-or amend last commit if you have single commit in your Pull Request:
+or amend your last commit if you have a single commit in your Pull Request:
 
 ```shell
 $ git commit --amend -s