Add github input plugin (#5587)

plugins/inputs/github/README.md

@@ -0,0 +1,47 @@
+# GitHub Input Plugin
+
+The [GitHub](https://www.github.com) input plugin gathers statistics from GitHub repositories.
+
+### Configuration:
+
+```toml
+[[inputs.github]]
+  ## List of repositories to monitor
+  ## ex: repositories = ["influxdata/telegraf"]
+  # repositories = []
+
+  ## Optional: Unauthenticated requests are limited to 60 per hour.
+  # access_token = ""
+
+  ## Optional: Default 5s.
+  # http_timeout = "5s"
+```
+
+### Metrics:
+
+- github_repository
+  - tags:
+    - `name` - The repository name
+    - `owner` - The owner of the repository
+    - `language` - The primary language of the repository
+    - `license` - The license set for the repository
+  - fields:
+    - `stars` (int)
+    - `forks` (int)
+    - `open_issues` (int)
+    - `size` (int)
+
+* github_rate_limit
+  - tags:
+    - `access_token` - An obfusticated reference to the configured access token or "Unauthenticated"
+  - fields:
+    - `limit` - How many requests you are limited to (per hour)
+    - `remaining` - How many requests you have remaining (per hour)
+    - `blocks` - How many requests have been blocked due to rate limit
+
+### Example Output:
+
+```
+github,full_name=influxdata/telegraf,name=telegraf,owner=influxdata,language=Go,license=MIT\ License stars=6401i,forks=2421i,open_issues=722i,size=22611i 1552651811000000000
+internal_github,access_token=Unauthenticated rate_limit_remaining=59i,rate_limit_limit=60i,rate_limit_blocks=0i 1552653551000000000
+```