Created Tests (markdown)

Tests.md

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+There are unit tests in `tests` and eslint. They're run each time you push on [CircleCI](https://travis-ci.org/BoostIO/Boostnote).
+
+# Run test
+Execute them.
+
+```sh
+$ yarn run test
+$ yarn run lin
+```
+
+# Unit tests
+There are 2 types of test in Boostnote, one is unit tests for `dataApi`, another one is unit tests for other modules in `browser/lib/`.
+
+# Why no e2e test?
+Actually, it was. Spectron was used for e2e test tool. The reason why it was removed is that because it has problems below:
+1. Spectron doesn't support button click in a dialog box so the files for tests remain even after the tests finish
+2. It's hard to specify input field due to CodeMirror complication
+3. It doesn't run well on CI due to permission