[pkg/ottl] Added 'Adding new functions/converters' guidelines

[michalpristas]

Description:
As discussed here we should agree on some guidelines when it comes to creating proposals for new functions and converters

cc @TylerHelmuth

Link to tracking Issue: -

Testing: -

Documentation: -

[evan-bradley]

Thanks for adding this!

[evan-bradley]

Suggested change

	## Adding New Functions/Converters
	Before raising a PR with a new function or converter, raise a PR to verify its acceptance. While acceptance is strongly specific to a specific use case, consider these guidelines for early assessment.
	## Adding New Editors/Converters
	Before raising a PR with a new Editor or Converter, raise an issue to describe the proposed function. While acceptance strongly depends on factors specific to each proposal, consider these guidelines for early assessment.

[TylerHelmuth]

@djaglowski I'm very interested in your opinion here, as new functions are similar to new stanza operators.

[djaglowski]

These guidelines seem reasonable to me. I think they leave room for interpretation, which is good because in some cases we may find other factors to be important.

[evan-bradley]

Thanks @michalpristas.