Added a short notice if the server is used interactively

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)

What kind of change does this PR introduce?
If someone tries to start the language server interactively a warning is issued:

arduino-language-server is a language server that provides IDE-like features to editors.

It should be used via an editor plugin rather than invoked directly. For more information, see:
https://github.com/arduino/arduino-language-server/
https://microsoft.github.io/language-server-protocol/

What is the current behavior?
The language server starts without any visible output.

What is the new behavior?
The language server prints a warning.