Update README.md

README.md

@@ -288,7 +288,7 @@ db.remove({ system: 'solar' }, { multi: true }, function (err, numRemoved) {
 ### Indexing
 NeDB supports indexing. It gives a very nice speed boost and can be used to enforce a unique constraint on a field. You can index any field, including fields in nested documents using the dot notation. For now, indexes are only used to speed up basic queries and queries using `$in`, `$lt`, `$lte`, `$gt` and `$gte`.
 
-To create an index, use `datastore.ensureIndex(options, cb)`, where callback is optional and get passed an error if any. The options are:  
+To create an index, use `datastore.ensureIndex(options, cb)`, where callback is optional and get passed an error if any (usually a unique constraint that was violated). `ensureIndex` can be called when you want, even after some data was inserted, though it's best to call it at application startup. The options are:  
 
 * **fieldName** (required): name of the field to index. Use the dot notation to index a field in a nested document.
 * **unique** (optional, defaults to `false`): enforce field uniqueness. Note that a unique index will raise an error if you try to index two documents for which the field is not defined.