Add an importer doc for firestore adaptor

docs/importer/adaptors/firestore.md

+# Cloud Firestore
+
+- The [firestore](https://firebase.google.com/docs/firestore) adaptor is capable of reading collections from the firestore database.
+
+- Checkout firestore [service accounts](https://console.cloud.google.com/iam-admin/serviceaccounts) for additional 
+details about initializing the firestore SDK on your own server.  
+
+Here is how a configuration file looks like:
+
+```ini
+src_type=firestore
+sac_path="/path/to/service_account_credentials_file.json"
+src_filter="<collection name/regex>"
+
+dest_type=elasticsearch
+dest_uri=appname
+```
+
+## Usage
+`abc import --src_type=firestore --sac_path="/path/to/service_account_credentials_file.json" "<destination url>"`
+
+## Filter the collections
+`abc import --src_type=firestore --sac_path="/path/to/service_account_credentials_file.json" --src_filter="<collection name/regex>" "<destination url>"`
+
+## Example
+`abc import --src_type=firestore --sac_path="/home/johnappleseed/ServiceAccountKey.json" appbase-firestore-demo`
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