palomar: add second README with ES ops stuff

cmd/palomar/README.opensearch.md

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+
+# Basic OpenSearch Operations
+
+We use OpenSearch version 2.5, with the built-in `analysis-icu`.
+
+If you are trying to use Elasticsearch 7.10, you can install the plugin with:
+
+    sudo /usr/share/elasticsearch/bin/elasticsearch-plugin install analysis-icu
+    sudo service elasticsearch restart
+
+## Local Development
+
+Run OpenSearch locally with docker:
+
+    docker run -p 9200:9200 -p 9600:9600 -e "discovery.type=single-node" opensearchproject/opensearch:2.5.0
+
+Eg, with OpenSearch running locally. Drop and rebuild the schema:
+
+    http delete :9200/palomar_post
+    http delete :9200/palomar_profile
+    http put :9200/palomar_post?include_type_name=true < palomar_post.json
+    http put :9200/palomar_profile?include_type_name=true < palomar_profile.json
+
+Put a single object (good for debugging):
+
+    head -n1 examples.json | http post :9200/palomar_post/_doc/0
+    http get :9200/palomar_post/_doc/0
+
+Bulk insert from a file on disk:
+
+    esbulk -verbose -id ident -index palomar_post -type _doc examples.json
+
+## Index Aliases
+
+To make re-indexing and schema changes easier, we can create versioned (or
+time-stamped) elasticsearch indexes, and then point to them using index
+aliases. The index alias updates are fast and atomic, so we can slowly build up
+a new index and then cut over with no downtime.
+
+    http put :9200/palomar_post_v04 < post_schema.json
+
+To do an atomic swap from one alias to a new one ("zero downtime"):
+
+    http post :9200/_aliases << EOF
+        {
+            "actions": [
+                { "remove": { "index": "palomar_post_v05", "alias": "palomar_post" }},
+                { "add":    { "index": "palomar_post_v06", "alias": "palomar_post" }}
+            ]
+        }
+    EOF
+
+To replace an existing ("real") index with an alias pointer, do two actions
+(not truly zero-downtime, but pretty fast):
+
+    http delete :9200/palomar_post
+    http put :9200/palomar_post_v03/_alias/palomar_post
+
+## Full-Text Querying
+
+A generic full-text "query string" query look like this (replace "blood" with
+actual query string, and "size" field with the max results to return):
+
+    GET /palomar_post/_search
+    {
+      "query": {
+        "query_string": {
+          "query": "blood",
+          "analyzer": "textIcuSearch",
+          "default_operator": "AND",
+          "analyze_wildcard": true,
+          "lenient": true,
+          "fields": ["handle^5", "text"]
+        }
+      },
+      "size": 3
+    }
+
+In the results take `.hits.hits[]._source` as the objects; `.hits.total` is the
+total number of search hits.
+