README.md: add sync mode requirements for issue #20260

[lhendre]

Adding in missing syncmode documentation as requested in issue #20260

Added in full, fast and snap with flag examples. Based on information provided in the Ethereum blog here https://blog.ethereum.org/2021/03/03/geth-v1-10-0/

[s1na]

The ticket you referenced is about missing pages on the website. If you'd like to contribute to a page about sync modes please see the branch gh-pages where our website is hosted.

Also note that fast sync mode is deprecated and not shipped with the most recent geth release.

My 2cents is that the README is already overloaded and we don't really need to add this there. But will wait for other people to chime in.

[lhendre]

I read it as the README/the readme was acceptable given #24395, that being said can move or drop the pr so will wait for others to chime in or move it based on your feedback if they dont.

Ill leave fast sync but add in that information if that works

[holiman]

I don't think it's a good idea to mix in things like how long it takes, which differs a lot across hardware and networks and locations.
I also don't agree with the descriptions of the trust model(s), for fast-sync (which is the same as snap sync).

[ligi]

The content is also on the website - let's not dup it: ethereum/ethereum-org-website#3885