update README with LUMIGO_PROPAGATE_W3C [skip ci]

Author: saartochner-lumigo

README.md

@@ -52,6 +52,7 @@ def my_lambda(event, context):
 * `LUMIGO_DEBUG=TRUE` - Enables debug logging
 * `LUMIGO_SECRET_MASKING_REGEX=["regex1", "regex2"]` - Prevents Lumigo from sending keys that match the supplied regular expressions. All regular expressions are case-insensitive. By default, Lumigo applies the following regular expressions: `[".*pass.*", ".*key.*", ".*secret.*", ".*credential.*", ".*passphrase.*"]`. 
 * `LUMIGO_DOMAINS_SCRUBBER=[".*secret.*"]` - Prevents Lumigo from collecting both request and response details from a list of domains. This accepts a comma-separated list of regular expressions that is JSON-formatted. By default, the tracer uses `["secretsmanager\..*\.amazonaws\.com", "ssm\..*\.amazonaws\.com", "kms\..*\.amazonaws\.com"]`. **Note** - These defaults are overridden when you define a different list of regular expressions.
+* `LUMIGO_PROPAGATE_W3C=TRUE` - Add W3C TraceContext headers to outgoing HTTP requests. This enables uninterrupted transactions with applications traced with OpenTelemetry.
 * `LUMIGO_SWITCH_OFF=TRUE` - In the event a critical issue arises, this turns off all actions that Lumigo takes in response to your code. This happens without a deployment, and is picked up on the next function run once the environment variable is present.
 
 ### Step Functions