Document our processors.

README.md

@@ -19,25 +19,45 @@ See [the module's docs for more info](modules/migrate_embargoes_to_embargo/READM
 Configuration options can be set at `admin/config/content/embargo`,
 including a contact email and notification message.
 
-Embargoes can be managed at `admin/content/embargo`. 
+Embargoes can be managed at `admin/content/embargo`.
 
 To add an IP range for use on embargoes, navigate to
 `admin/content/embargo/range` and click 'Add IP range'. Ranges
 created via this method can then be used as IP address whitelists when creating
-embargoes. This [CIDR to IPv4 Conversion utility](https://www.ipaddressguide.com/cidr) 
+embargoes. This [CIDR to IPv4 Conversion utility](https://www.ipaddressguide.com/cidr)
 can be helpful in creating valid CIDR IP ranges.
 
+### `search_api` processor(s)
+
+We have multiple `search_api` processors which attempt to constrain search
+results based on the effects of embargoes on the entities represented by search
+results, including:
+
+- `embargo_processor` ("Embargo access (deprecated)")
+  - Adds additional properties to the indexed rows, requiring additional index
+    maintenance on mutation of the entities under consideration, but should
+    theoretically work with any `search_api` backend
+- `embargo_join_process` ("Embargo access, join-wise")
+  - Requires Solr/Solarium-compatible index, and indexing of embargo entities in
+    the same index as the node/media/files to be search, tracking necessary info
+    and performing
+    [Solr joins](https://solr.apache.org/guide/solr/latest/query-guide/join-query-parser.html)
+    to constrain results
+
+Typically, only one should be used in any particular index.
+
 ## Usage
 
 ### Applying an embargo
 
-An embargo can be applied to an existing node by clicking the 
-"Embargoes" tab on a node, or navigating to 
+An embargo can be applied to an existing node by clicking the
+"Embargoes" tab on a node, or navigating to
 `embargoes/node/{node_id}`. From here, an embargo can be applied if it doesn't
 already exist, and existing embargoes can be modified or removed.
 
 ## Known Issues
-Embargoed items may show up in search results. To work around this at a cost to performance you can enable access checking in your search views.
+
+- Embargoed items show up in search results: Enable one of our `search_api` processors to handle applying embargo restrictions.
 
 ## Troubleshooting/Issues