update for npm info

- add info that you install the script via npm
- add info about needing Node.js
- add intro line about which extension the script is for
- general style updates

Author: rachelsaunders

firestore-bigquery-export/guides/IMPORT_EXISTING_DOCUMENTS.md

@@ -1,3 +1,5 @@
+The `fs-bq-import-collection` script is for use with the official Firebase Extension [**Export Collections to BiqQuery**](https://github.com/firebase/extensions/tree/master/firestore-bigquery-export).
+
 ### Overview
 
 The import script (`fs-bq-import-collection`) can read all existing documents in a Cloud Firestore collection and insert them into the raw changelog table created by the Export Collections to BigQuery extension. The script adds a special changelog for each document with the operation of `IMPORT` and the timestamp of epoch. This ensures that any operation on an imported document supersedes the import record.
@@ -6,22 +8,28 @@ You may pause and resume the script from the last batch at any point.
 
 #### Important notes
 
-+   Run the script over the entire collection **_after_** installing the Export Collections to BigQuery extension; otherwise the writes to your database during the import might not be exported to the dataset.
++   You must run the script over the entire collection **_after_** installing the Export Collections to BigQuery extension; otherwise the writes to your database during the import might not be exported to the dataset.
 +   The import script can take up to _O(collection size)_ time to finish. If your collection is large, you might want to consider [loading data from a Cloud Firestore export into BigQuery](https://cloud.google.com/bigquery/docs/loading-data-cloud-firestore).
-+   You will see redundant rows in your raw changelog table:
++   You will see redundant rows in your raw changelog table if either of the following happen:
 
     +   If document changes occur in the time between installing the extension and running this import script.
     +   If you run the import script multiple times over the same collection.
 
 ### Install and run the script
 
-This import script uses several values from your installation of the extension:
+Install the import script using `npm` (the Node Package Manager). Note that the import script uses several values from your installation of the extension:
 
 +   `${PROJECT_ID}`: the project ID for the Firebase project in which you installed the extension
 +   `${COLLECTION_PATH}`: the collection path that you specified during extension installation
 +   `${DATASET_ID}`: the ID that you specified for your dataset during extension installation
 
-1.  Run `npx @firebaseextensions/fs-bq-import-collection`.
+1.  Make sure that you have [Node.js](https://www.nodejs.org/) installed in order to access the `npm` command tools.
+
+1.  Install the import script via `npm` by running the following command:
+
+    ```
+    npx @firebaseextensions/fs-bq-import-collection
+    ```
 
 1.  When prompted, enter the Cloud Firestore collection path that you specified during extension installation, `${COLLECTION_PATH}`.