add circleci info

CircleCI.md

@@ -1 +1,49 @@
-x
+Shellcheck can be run on CircleCI in a variety of ways:
+
+### Via [CircleCI Orbs](http://circleci.com/orbs)
+CircleCI provides a `circleci/shellcheck` orb that can be used in any CircleCI workflow:
+
+https://circleci.com/orbs/registry/orb/circleci/shellcheck
+
+```yaml
+version: 2.1
+
+orbs:
+  shellcheck: circleci/shellcheck@1.0.0
+
+workflows:
+  shellcheck:
+    jobs:
+      - shellcheck/check
+```
+
+### Manually
+Simply copy and paste the [existing installation instructions](https://github.com/koalaman/shellcheck#installing) into a `run` step in your CircleCI job.
+
+For example:
+
+```yaml
+version: 2.1
+
+jobs:
+  build:
+    macos:
+      xcode: "9.0"
+    steps:
+      - checkout
+      - run: brew install shellcheck
+```
+
+Or, in a Linux-based image:
+
+```yaml
+version: 2.1
+
+jobs:
+  build:
+    machine: true
+    steps:
+      - checkout
+      - run: apt-get install shellcheck
+```
+