Add another blank line here.

I'd change the "Installation" subheading to "Installing from git" and then get rid of this line.

Typos:

navigated -> navigate

dirctory -> directory
This probably isn't necessary at all, though. People working with this repo should probably already have the necessary skills to be in a terminal in their git clone.

Sentences should start with capital letters. I think the link should also be inline here:

Install [gulp](https://gulpjs.com) by running:

If instead we suggest doing sude npm install -g gulp, then below you can just tell people to run gulp instead of ./node_modules/.bin/gulp

Typos:

Seconly -> Secondly

installed -> install
Line should also end in a colon.

Should start with a capital letter and end in a color. Also, typo: followinng

Space between "browser" and "(". Should also end in a period.

These should probably all be in one paragraph.

Where exactly in the UI is "customise and control Google Chrome"? I can't find it. There's also some trailing whitespace on this line and the next.

next -> Next

ur -> your

Period at the end of this sentence.

you -> You

being installed on -> in

Trailing whitespace at the end of this line.

Summary should start with a capital letter.

What about Firefox instructions?

Blank line after this.

No space before the :.

No space before the :.

No space before the :. Also theree's an extra space character at the end of the line.
Maybe have this say "To build the extension, run:"

Don't need this--you're telling where the .build directory is later in there.

No space before the period.