Add ElasticSearch query examples

api/internal/crawl/search_cmds/fieldExistence.md

@@ -0,0 +1,55 @@
+Count the documents whose `document` field is empty (The reason why the `document` field
+of a document is empty is because of empty documents):
+```
+curl -X GET "${ElasticSearchURL}:9200/kustomize/_search?pretty" -H 'Content-Type: application/json' -d'
+{
+   "size": 10000,
+    "query": {
+        "bool": {
+            "must_not": {
+                "exists": {
+                    "field": "document"
+                }
+            }
+        }
+    }
+}
+'
+```
+
+Find all the documents having the `creationTime` field set:
+```
+curl -X GET "${ElasticSearchURL}:9200/kustomize/_search?pretty" -H 'Content-Type: application/json' -d'
+{
+    "query": {
+        "exists": {
+            "field": "creationTime"
+        }
+    }
+}
+'
+```
+
+Find all the documents whose `creationTime` field is not set:
+```
+curl -X GET "${ElasticSearchURL}:9200/kustomize/_search?pretty" -H 'Content-Type: application/json' -d'
+{
+   "size": 10000,
+    "query": {
+        "bool": {
+            "must_not": {
+                "exists": {
+                    "field": "creationTime"
+                }
+            }
+        }
+    }
+}
+'
+```
+
+The following fields of a document in the kustomize index are always non-empty:
+`repositoryUrl`, `filePath`, `defaultBranch`.
+
+The following fields of a document in the kustomize index may be empty:
+`kinds`, `identifiers`, `values`.