Merge pull request #972 from wiktorn/readme_update

Add note about TF_PLUGIN_CACHE_DIR

CONTRIBUTING.md

@@ -71,6 +71,16 @@ pytest tests/examples
 
 Once everything looks good, add/commit any pending changes then push and open a PR on GitHub. We typically enforce a set of design and style conventions, so please make sure you have familiarized yourself with the following sections and implemented them in your code, to avoid lengthy review cycles.
 
+HINT: if you work on high-latency or low-bandwidth network use `TF_PLUGIN_CACHE_DIR` environment variable to dramatically speed up the tests, for example:
+```bash
+TF_PLUGIN_CACHE_DIR=/tmp/tfcache pytest tests
+```
+
+Or just add into your [terraformrc](https://developer.hashicorp.com/terraform/cli/config/config-file):
+```
+plugin_cache_dir = "$HOME/.terraform.d/plugin-cache"
+```
+
 ## Developer's handbook
 
 Over the years we have assembled a specific set of design principles and style conventions that allow for better readability and make understanding and changing code more predictable.