Add instuction to contributers for pre-commit about terraform-aws-github-runner

[axel3rd]

The Submitting a Merge Request section is clear (Installing terraform-docs & tflint is quite easy).

The pre-commit run --all-files is simple to use to check manually before or after some bug/feature fix.

Perhaps one attention about Terraform docs phase, it changes Markdown content adding many links like (Sample from git diff):

 ## Modules

 | Name | Source | Version |
 |------|--------|---------|
-| runner_binaries | ./modules/runner-binaries-syncer |  |
-| runners | ./modules/runners |  |
-| ssm | ./modules/ssm |  |
-| webhook | ./modules/webhook |  |
+| <a name="module_runner_binaries"></a> [runner\_binaries](#module\_runner\_binaries) | ./modules/runner-binaries-syncer | n/a |
+| <a name="module_runners"></a> [runners](#module\_runners) | ./modules/runners | n/a |
+| <a name="module_ssm"></a> [ssm](#module\_ssm) | ./modules/ssm | n/a |
+| <a name="module_webhook"></a> [webhook](#module\_webhook) | ./modules/webhook | n/a |

I don't know if it is really wanted, perhaps depends of terraform-docs version (v0.15.0 is used).

[npalm]

Instructions are available