Add note in CI.md about reduced tests on AppVeyor

Author: YannickJadoul

CI.md

@@ -1,11 +1,12 @@
 This is a summary of the Python versions and platforms covered by the different CI platforms:
 
-|          | 3.5              | 3.6              | 3.7                                               | 3.8              |
-|----------|------------------|------------------|---------------------------------------------------|------------------|
-| Linux    | Travis CI        | CircleCI         | AppVeyor / GitHub Actions                         | Azure Pipelines  |
-| macOS    | Azure Pipelines  | CircleCI         | AppVeyor / Travis CI¹ / CircleCI / GitHub Actions | Azure Pipelines  |
-| Windows  | TravisCI         | Azure Pipelines  | AppVeyor / GitHub Actions                         | Azure Pipelines  |
+|          | 3.5              | 3.6              | 3.7                                                | 3.8              |
+|----------|------------------|------------------|----------------------------------------------------|------------------|
+| Linux    | Travis CI        | CircleCI         | AppVeyor² / GitHub Actions                         | Azure Pipelines  |
+| macOS    | Azure Pipelines  | CircleCI         | AppVeyor² / Travis CI¹ / CircleCI / GitHub Actions | Azure Pipelines  |
+| Windows  | TravisCI         | Azure Pipelines  | AppVeyor² / GitHub Actions                         | Azure Pipelines  |
 
 > ¹ Python version not really pinned, but dependent on the (default) version of image used.
+> ² AppVeyor only runs the "basic" test to reduce load.
 
 Non-x86 architectures are covered on Travis CI using Python 3.5.