Update readme to describe building

[nekonenene]

Proposed changes

Added a description of how to build. It will help many developers who wants to get the latest materialize.css or materialize.js.

I'm not good at English, so please fix more natural if you feel something wrong.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[DanielRuf]

Please revert the version changes in package.json.

[DanielRuf]

hmm I got what newver arg means, but how about oldver? Doesn't seem to affect anything? 😕

@Smankusors oldver is the old version string that is replaced by gulp-replace, just a simple find and replace. We could make it optional in the future by reading package.json and what I like is also yarn with its version --minor command.