add plugin-index doc

DOCS.md

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+---
+date: 2018-12-09T00:00:00+00:00
+title: Coveralls
+author: lizheming
+tags: [ coverage, test, reporting, coveralls ]
+repo: lizheming/drone-coveralls
+logo: coveralls.svg
+image: lizheming/drone-coveralls
+---
+This plugin can upload coverage reports to [Coveralls](https://coveralls.iio). The below pipeline configuration demonstrates simple usage:
+
+```yaml
+steps:
+- name: test
+  image: node:alpine
+  commands:
+  - npm install
+  - npm run test -- --reporter=text-lcov > ./lcov.info
+
+- name: coveralls
+  image: lizheming/drone-coveralls
+  environment:
+    COVERALLS_REPO_TOKEN:
+      from_secret: coveralls_token
+  settings:
+    files:
+    - ./lcov.info
+```
+
+# Parameter Reference
+
+files
+: list of target files to upload. Required.
+corpid
+
+token
+: if you have not set environment `COVERALLS_REPO_TOKEN`, you should set the private repository token.
+
+debug
+: debug mode, defaults to false.