docs: add warning about importance of option order

[Rabsztok]

If you place Template() methods after Directory() method, then templating won't be applied resulting in error. This commit adds a very short warning message in readme about that behaviour. I found out about it in following discussion: https://github.com/go-testfixtures/testfixtures/issues/60

[slsyy]

@Rabsztok IMO it makes more sense to fix this behavior instead of documenting it

Let me check, if it is possible. I will ping you