docs: show how you can get the types to work in a commonjs module

[joshstevens19]

Description

A issue was raised a few weeks ago about how to get the typing's to work in a commonjs module.

#2486

I have now added instructions in the README.md so others can easily find how to get it working.

Type of change

• Bug fix
• New feature
• Breaking change
• Enhancement

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage remained the same at 98.651% when pulling 4183cdc on joshstevens19:update-readme-to-get-types-with-commonjs into 1a7c600 on ethereum:1.0.