Add .md file to docs folder for new rule: azurerm_storage_account_inv…

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docs/rules/azurerm_storage_account_invalid_name.md

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+# azurerm_storage_account_invalid_name
+
+Warns about values that appear to be invalid based on [azure-rest-api-specs](https://github.com/Azure/azure-rest-api-specs).
+
+In this rule, the string must match the regular expression `^[a-z0-9]{3,24}$``.
+
+## Example
+
+```hcl
+resource "azurerm_storage_account" "foo" {
+  name = ... // invalid value
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Error: "..." does not match valid pattern ^[a-z0-9]{3,24}$ (azurerm_storage_account_invalid_name)
+
+  on template.tf line 15:
+  15:   name = ... // invalid value
+
+Reference: https://github.com/terraform-linters/tflint-ruleset-azurerm/blob/v0.4.0/docs/rules/azurerm_storage_account_invalid_name.md
+
+```
+
+## Why
+
+Requests containing invalid values will return an error when calling the API by `terraform apply`.
+
+## How to Fix
+
+Replace the warned value with a valid value.
+
+## Source
+
+https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/resource-name-rules#microsoftstorage