Add protocol documentation (#3)

PROTOCOL.md

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+# Browserpass Communication Protocol
+This document describes the protocol used for communication between the browser extension,
+and the native host application.
+
+## Response Types
+
+### OK
+
+Consists solely of an `ok` status, an integer app version, and a `response` field. The response
+may be of any type.
+
+The app version is an integer, calculated by `(MAJOR * 1000000) + (MINOR * 1000) + PATCH`.
+
+```
+{
+    “status”: “ok”,
+    “version”: <int>
+    “response”: <any type>
+}
+```
+
+### Error
+
+Consists solely of an `error` status, a nonzero integer error code, and an optional `params`
+object that provides any parameters that should accompany the error.
+
+```
+{
+    “status”: “error”,
+    “code”: <int>
+    “params”: {
+       “<paramN>”: <valueN>
+    }
+}
+```
+
+When the error message is relaying a message from a native system component, then that message
+should be supplied as a `message` parameter.
+
+## List of Error Codes
+
+| Code | Description | Parameters |
+| ---- | ----------- | ---------- |
+|      |             |            |
+
+## Settings
+
+The `settings` object is a key / value map of individual settings. It's provided by the
+browser to the native app as part of every request.
+
+Settings are saved in browser local storage. Each top-level setting is saved separately,
+JSON-encoded and saved by its key.
+
+Settings may also be supplied via a `.browserpass.json` file in the root of a password store,
+and via parameters in individual `*.gpg` files.
+
+Settings are applied using the following priority, highest first:
+
+  1. Configured by the user in specific `*.gpg` files (e.g. autosubmit: true)
+  2. Configured by the user via the extension options
+  3. Configured by the user in each store’s `.browserpass.json` file
+  4. Defaults shipped with the browser extension
+
+### Global Settings
+
+| Setting | Description                                          | Default |
+| ------- | ---------------------------------------------------- | ------- |
+| gpgPath | Optional path to gpg binary                          | `null`  |
+| stores  | List of password stores with store-specific settings | `null`  |
+
+### Store-specific Settings
+
+| Setting | Description                                          | Default |
+| ------- | ---------------------------------------------------- | ------- |
+|         |                                                      |         |
+
+## Actions
+
+### Configure
+
+Returns a response containing the per-store config. Used to check that the host app
+is alive, determine the version at startup, and provide per-store defaults.
+
+#### Request
+
+```
+{
+    “settings”: <settings object>,
+    “action”: “configure”
+}
+```
+
+#### Response
+
+```
+{
+
+    “status”: “ok”,
+    “version”: <int>
+    “response”: {
+        “storeSettings”: {
+            “storeName”: <raw contents of storePath/.browserpass.json>
+        }
+    }
+}
+```
+
+### List
+
+Get a list of all `*.gpg` files for each of a provided array of directory paths. The `storeN`
+is the name of a password store, the key in `“settings.stores”` object.
+
+#### Request
+
+```
+{
+    “settings”: <settings object>,
+    “action”: “list”
+}
+```
+
+#### Response
+
+```
+{
+    “status”: “ok”,
+    “version”: <int>,
+    “response”: {
+        “files”: {
+            “storeN”: [<storeNPath/file1.gpg>, <...>],
+            “storeN+1”: [<storeN+1Path/file1.gpg>, <...>]
+        }
+    }
+}
+```
+
+### Fetch
+
+Get the decrypted contents of a specific file.
+
+#### Request
+
+```
+{
+    “settings”: <settings object>,
+    “action”: “fetch”,
+    “store”: <storeName>
+    “file”: “relative/path/to/file.gpg”
+}
+```
+
+#### Response
+
+```
+{
+    “status”: “ok”,
+    “version”: <int>,
+    “response”: {
+        “data”: <decrypted file contents>
+    }
+}
+```