update readme

Author: nicpottier

Diff for: README.md

@@ -1,2 +1,49 @@
-# rp-indexer
-Simple service for indexing RapidPro contacts into ElasticSearch
+# RapidPro Indexer
+
+Simple service for indexing RapidPro contacts into ElasticSearch.
+
+This service can run in two modes:
+
+1) the default mode, which simply queries the ElasticSearch database, finds the most recently
+modified contact, then on a schedule queries the `contacts_contact` table on the RapidPro
+database for contacts to add or delete. You should run this as a long running service which
+constantly keeps ElasticSearch in sync with your RapidPro contacts.
+
+2) a rebuild mode, started with `--rebuild`. This builds a brand new index from nothing, querying
+all contacts on RapidPro. Once complete, this switches out the alias for the contact index
+with the newly build index. This can be run on a cron (in parallel with the mode above) to rebuild
+your index occasionally to get rid of bloat.
+
+## Usage
+
+It is recommended to run the service with two environment variables set:
+
+ * `INDEXER_DB`: a URL connection string for your RapidPro database
+ * `INDEXER_ELASTIC_URL`: the URL for your ElasticSearch endpoint
+
+```
+Indexes RapidPro contacts to ElasticSearch
+
+Usage of indexer:
+  -db string
+        the connection string for our database (default "postgres://localhost/rapidpro")
+  -debug-conf
+        print where config values are coming from
+  -elastic-url string
+        the url for our elastic search instance (default "http://localhost:9200")
+  -help
+        print usage information
+  -index string
+        the alias for our contact index (default "contacts")
+  -poll int
+        the number of seconds to wait between checking for updated contacts (default 5)
+  -rebuild
+        whether to rebuild the index, swapping it when complete, then exiting (default false)
+
+Environment variables:
+                                  INDEXER_DB - string
+                         INDEXER_ELASTIC_URL - string
+                               INDEXER_INDEX - string
+                                INDEXER_POLL - int
+                             INDEXER_REBUILD - bool
+```