feature: the key of a credential store may be a path of keys, so stores of several kinds share one object of config.json.

README.md

@@ -124,7 +124,19 @@ saved or used; a secret encrypted with another key cannot be decrypted and has t
 again.
 
 `CredentialStore` describes a store: the path of `config.json`, the key, the kind, the reserved
-names and whether a missing `config.json` reads as `{}`.
+names and whether a missing `config.json` reads as `{}`. The key is a key of `config.json` or a
+path of keys separated by dots, such as `connections.pg`, for a list in nested objects; a write
+creates the objects of the path that are missing and keeps their other keys, so stores of several
+kinds can share one object:
+
+```json
+{
+  "connections": {
+    "pg": [{ "name": "state", "host": "db.example.com", "port": 5432, "...": "..." }],
+    "mssql": [{ "name": "warehouse", "host": "sql.example.com", "port": 1433, "...": "..." }]
+  }
+}
+```
 
 - `credentialProblems(value, store)`: the problems of the key's value by `<key>.<index>.<field>`,
   for the application's own check of `config.json`.