Added section on index build time [skip ci]

README.md

@@ -190,6 +190,22 @@ CREATE INDEX ON items USING hnsw (embedding vector_cosine_ops);
 
 Vectors with up to 2,000 dimensions can be indexed.
 
+### Index Build Time
+
+Indexes build significantly faster when the graph fits into `maintenance_work_mem`
+
+```sql
+SET maintenance_work_mem = '8GB';
+```
+
+A notice is shown when the graph no longer fits
+
+```text
+NOTICE:  hnsw graph no longer fits into maintenance_work_mem after 100000 tuples
+DETAIL:  Building will take significantly more time.
+HINT:  Increase maintenance_work_mem to speed up builds.
+```
+
 ### Index Options
 
 Specify HNSW parameters