Added sections to README, renamed `LinearGradient` & fixed `secenegra…

…ph.selection` type

- fixes #72
- fixes #60
- closes #41

- Increased version number in package.json

v20.1 – Merge pull request #51 from AdobeXD/uxp-options-fix

Moved from JSDoc to type declarations for options objects

Merge pull request #47 from AdobeXD/xd-19-fixes

XD 20 APIs

Merge pull request #46 from AdobeXD/xd-19

XD 19 APIs update

Fixed a few issues

Merge pull request #40 from AdobeXD/xd-15

Xd 15

Fixing typo in class name

Merge pull request #39 from AdobeXD/xd-14-apis

Xd 14 apis

Updated scenegraph.Text docs