Add build instructions

[eljamm]

Description

• Build anki-review-heatmap.js from source
• Add build instructions to README.md

Closes #197

Checklist:

Please replace the space inside the brackets with an x and fill out the ellipses if the following items apply:

• I've read and understood the contribution guidelines
• I've tested my changes against at least one of the following Anki builds:
  • Latest standard Anki 2.1 binary build
  • Latest alternative Anki 2.1 binary build
• I've tested my changes on at least one of the following platforms:
  • Linux, version:
  • Windows, version:
  • macOS, version:
• My changes potentially affect non-desktop platforms, of which I've tested:
  • AnkiMobile, version:
  • AnkiDroid, version:
  • AnkiWeb

[olliecheng]

Apologies for the misnamed commit (should be "function" not "library") - it's 3AM and I got caught up in an interesting problem! The patch should be stable and I've tested it to be fine but make sure to use at your own risk. There's an explanation of how the bug occurs over in issue 214. Night everyone!