add note to constraints files (GoogleCloudPlatform#6758)

Author: leahecole

composer/airflow_1_samples/requirements.txt

@@ -1,3 +1,5 @@
+# be sure to update the constraints file to match
+# see https://airflow.apache.org/docs/apache-airflow/stable/installation/installing-from-pypi.html#constraints-files
 apache-airflow[gcp]==1.10.15
 apache-airflow-backport-providers-google==2021.3.3
 apache-airflow-backport-providers-cncf-kubernetes==2021.3.3

composer/blog/gcp-tech-blog/data-orchestration-with-composer/requirements.txt

@@ -1,3 +1,5 @@
+# be sure to update the constraints file to match
+# see https://airflow.apache.org/docs/apache-airflow/stable/installation/installing-from-pypi.html#constraints-files
 apache-airflow[google]==2.0.1
 apache-airflow-providers-apache-beam==3.0.1
 apache-airflow-providers-slack==4.0.1

composer/blog/gcp-tech-blog/unit-test-dags-cloud-build/requirements.txt

@@ -1,2 +1,4 @@
+# be sure to update the constraints file to match
+# see https://airflow.apache.org/docs/apache-airflow/stable/installation/installing-from-pypi.html#constraints-files
 apache-airflow[gcp]==1.10.14
 apache-airflow-backport-providers-google==2021.3.3

composer/workflows/requirements.txt

@@ -1,3 +1,5 @@
+# be sure to update the constraints file to match
+# see https://airflow.apache.org/docs/apache-airflow/stable/installation/installing-from-pypi.html#constraints-files
 apache-airflow==2.1.2
 apache-airflow-providers-apache-beam==3.0.0
 apache-airflow-providers-google==5.1.0