🎉 Add MongoDB Source

Signed-off-by: fut <fut.wrk@gmail.com>

docs/integrations/sources/mongodb.md

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+# Mongodb 
+
+The MongoDB source supports Full Refresh and Incremental sync strategies.
+
+## Resulting schema 
+MongoDB does not have anything like table definition, thus we have to define column types from actual attributes and their values. Discover phase have two steps:
+
+### Step 1. Find all unique properties
+Connector runs the map-reduce command which returns all unique document props in the collection. Map-reduce approach should be sufficient even for large clusters.
+
+### Step 2. Determine property types
+For each property found, connector selects 10k documents from the collection where this property is not empty. If all the selected values have the same type - connector will set appropriate type to the property. In all other cases connector will fallback to `string` type.
+
+## Features
+
+| Feature | Supported |
+| :--- | :--- |
+| Full Refresh Sync | Yes |
+| Incremental - Append Sync | Yes |
+| Replicate Incremental Deletes | No |
+
+### Full Refresh sync
+Works as usual full refresh sync.
+
+### Incremental sync
+Cursor field can not be nested. Currently only top level document properties are supported.
+
+Cursor should **never** be blank. In case cursor is blank - the incremental sync results might be unpredictable and will totally rely on MongoDB comparison algorithm. 
+
+Only `datetime` and `integer` cursor types are supported. Cursor type is determined based on the cursor field name:
+
+* `datetime` - if cursor field name contains a string from: `time`, `date`, `_at`, `timestamp`, `ts`
+* `integer` - otherwise
+
+## Getting started
+This guide describes in details how you can configure MongoDB for integration with Airbyte.
+
+### Create users
+
+Run `mongo` shell, switch to `admin` database and create a `READ_ONLY_USER`. `READ_ONLY_USER` will be used for Airbyte integration. Please make sure that user has read-only privileges.
+
+```js
+mongo
+use admin;
+db.createUser({user: "READ_ONLY_USER", pwd: "READ_ONLY_PASSWORD", roles: [{role: "read", db: "TARGET_DATABASE"}]}
+```
+
+Make sure the user have appropriate access levels.
+
+### Configure application
+
+In case your application uses MongoDB without authentication you will have to adjust code base and MongoDB config to enable MongoDB authentication. **Otherwise your application might go down once MongoDB authentication will be enabled.**
+
+### Enable MongoDB authentication
+
+Open `/etc/mongod.conf` and add/replace specific keys:
+
+```yaml
+net:
+  bindIp: 0.0.0.0
+
+security:
+  authorization: enabled
+```
+
+Binding to `0.0.0.0` will allow to connect to database from any IP address.
+
+The last line will enable MongoDB security. Now only authenticated users will be able to access the database.
+
+### Configure firewall
+
+Make sure that MongoDB is accessible from external servers. Specific commands will depend on the firewall you are using (UFW/iptables/AWS/etc). Please refer to appropriate documentation.
+
+Your `READ_ONLY_USER` should now be ready for use with Airbyte.