docs(rule): Update RuleContext API (#355)
docs(rule): Update RuleContext API (#355)

* docs(rule): Update RuleContext API

Add [@textlint/get-config-base-dir](https://github.com/textlint/get-config-base-dir "@textlint/get-config-base-dir")

* Update rule.md

textlint/textlint,textlint/textlint,textlint/textlint,textlint/textlint