Document how to minimize css, js and images

[mhagnumdw]

Add step-by-step in README.md.

cd assets/
sudo npm install gulp-cli -g
npm install
# then run to minimize
gulp default

Also inform that all tasks can be seen in the gulpfile.js file.

ps: I thought about creating this issue because not everyone is familiar with it.

[sylhare]

Oh True, good idea 😅

Can I re-use that for the documentation?
I guess I could add some more steps, like you'd need to have also node installed.

[mhagnumdw]

Let me handle then. I can do a PR.

[sylhare]

Oh nice thanks ;)