Please update instructions for building dependencies

The new version of mozjpeg uses cmake.

https://github.com/imagemin/imagemin-mozjpeg/issues/37
https://github.com/mozilla/mozjpeg/issues/314

Your project is a way for people who are not very familiar with C to use all this.
Could you please update your readme? 

I used just 
```
cmake -G"Unix Makefiles"
make 
```
 But maybe for some other users it will be different.