Writing I2C library - Philips trademarks
IANAL but I've never heard of Philips (NXP or whoever) pursuing little guys over this. Somebody the size of Atmel, maybe, hence TWI. 

Suggest you search for he I2C manual, UM10204.pdf (currently at https://www.nxp.com/docs/en/user-guide/UM10204.pdf ) Its "legal" section is only 1 page, not exactly threatening as these things go, mostly disclaiming liability, though it does claim one I2C-Bus trademark.

My reading would be : 


somewhere prominent in the docs like "legal disclaimers" on page 1 (and especially in any licence agreement you have) include that trademark claim 



  I 2 C-bus — logo is a trademark of NXP Semiconductors N.V.



and in the technical section. where you descrieb the I2C interface, refer and/or link to the User Guide, UM10204.pdf (link above) which is useful anyway


Then it is clear that you are making to claim to the IP, and actually steering other people towards NXP, and what corporation would sue you for free advertising?