Add api usage instructions

README.md

 This is the Ruby on Rails application that powers [status.gitlab.com](https://status.gitlab.com/).
 This software is MIT Expat licensed.
+
+## API
+You can use the default json api by calling: [https://status.gitlab.com/status/api](https://status.gitlab.com/status/api)
+
+### JSON format
+```
+{
+"latency":[{"time":1469104321,"state":0,"value":"0.093852s"},...],
+"ssh response time":[{"time":1469104321,"state":0,"value":"0.211936s"},...],
+"http project response time":[{"time":1469104321,"state":0,"value":"0.727958s"},...],
+"http response time":[{"time":1469104321,"state":0,"value":"0.408527s"},...]
+}
+```
+
+### JSON attributes
+- **time:** epoch timestamp
+- **state:** 0 OK, 1 NOT OK
+- **value:** text
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