Clarify the level of ANS Forth compliance

[MaxBarraclough]

I suggest the documentation should indicate how well zForth complies with the ANS Forth standard.

Related: if zForth has good compliance, I suggest submitting it to the big list of Forths: ForthHub/discussion#19

[zevv]

Thanks, added to README.

I know almost nothing about what makes a Forth ANS compliant or compatible, and I'm not even sure I want to make the effort to find out :)