documentation for file connectors (#71)

* documentation for file connectors

* addressing reviews

pkg/plugins/file/README.md

@@ -0,0 +1,38 @@
+### Overview
+The Conduit File plugin provides both source and destination connectors,
+allowing for a file to be either a source or a destination in a Conduit
+pipeline.
+
+### How it works
+The Source connector listens for changes appended to the source file and 
+sends records with the changes.
+The Destination connector receives records and writes them to a file.
+
+#### Source
+The Source connector only cares to have a valid path, even if the file 
+doesn't exist, it will still run and wait until a file with the configured
+name is there, then it will start listening to changes and sending records.
+
+#### Destination
+The Destination connector will create the file if it doesn't exist, and 
+records with changes will be appended to the destination file when received.
+
+### How to build it
+Run `make build-file-plugin`.
+
+### Configuration
+
+| name | part of            | description         | required | default value |
+|------|--------------------|---------------------|----------|---------------|
+|`path`|destination, source |The path to the file |true      |               |
+
+### Limitations
+* The  Source connector only detects appended changes to the file, so it
+  doesn't detect deletes or edits.
+* The connectors can only access local files on the machine where Conduit
+  is running. So, running Conduit on a server means it can't access a file
+  on your local machine.
+* Currently, only works reliably with text files (may work with non-text
+  files, but not guaranteed)
+
+