Properly Documented Readme file is required.

CS issue: If the line declaring an array spans longer than 80 characters, each element should be broken into its own line.

Reviewed the patch for spelling and grammar. Everything looks good, but:

+++ b/README.md
@@ -0,0 +1,51 @@
+ * Install this module as you would normally install a contributed Drupal module.

Text should manually word-wrap within around 80 cols.
Also "Configuration" part is missing. The configuration section (required) is necessary even when little configuration is required.
I would add something like:
1. Navigate to Administration > Extend and enable the module.
2. Navigate to Administration > Structure > Block layout and "Place block" from "Financial Calculators" category.

It's better now. Thanks for the patch.