add new update docs

docs/installation.rst

@@ -41,3 +41,25 @@ Installation will enable offline usage of aggregated and refined financial
 features without internet access or API keys (the recommended way to explore
 data uninterrupted). See the :doc:`CLI docs <cli>` for more **finagg** CLI
 details.
+
+Updating for Faster Subsequent Installations
+--------------------------------------------
+
+Depending on the number of tickers you're aggregating data for, installation
+can take up to several hours. The Yahoo! Finance installation process is usually
+the main bottleneck (extensive stock histories can take a while to pull from APIs
+and can take a while to insert into local databases).
+
+You can speed up subsequent updates by updating Yahoo! Finance data rather than
+installing from scratch, keeping previously installed data and only installing
+new data. This can result in much faster subsequent installations. Instead of
+calling the normal installation CLI, you can instead do something like the
+following:
+
+.. code:: console
+
+    finagg yfinance update -a
+    finagg install -s yfinance -ss economic -ts indices -z -r
+
+This will update the Yahoo! Finance tables and then freshly install all the
+other tables, resulting in a much faster installation.