add documentation for insensitive setting

[michael-brueggemann]

In German, the a noun starts with an upper case (e.g. "Stadt" = "city").
In source code, a variable name normally starts with a lower case (=> "stadt"). Therefore the case sensitive setting should be disabled.
Please add in the documentation of this extension, that for German spell checking this setting is useful.

"cSpell.languageSettings": [
  {
    "locale": "de-de",
    "languageId": "*",
    "caseSensitive": false
  }
],