docs: add UPGRADE instructions

Author: mrexox

UPGRADE.md

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+# v1 -> v2
+
+Notable changes:
+
+- **v2** uses [universal coverage reporter](https://github.com/coverallsapp/coverage-reporter) instead of [node-coveralls](https://github.com/nickmerwin/node-coveralls) like in **v1**.
+
+- Option `path-to-lcov` is now deprecated, use `file` instead.
+
+- You can skip `file` option and coveralls will try to find all supported coverage files and combine their data.
+