Reorganize Eth1-specs Repo & Add Network Upgrades Info

[timbeiko]

Two big changes in this PR (I know...):

1. Change the top-level README & folder structure to make it clear that more than network upgrades spec are tracked here (specifically, the JSON RPC spec);
2. Clarify the CFI process by including some information from a previously opened PR.

Given that ethereum/pm#295 and #23 are still opened, I did not clarify where CFI is being tracked, or whether it resets every upgrade. Based on the outcome of these two issues, more changes will be necessary, but I think merging this is a net improvement.

[timbeiko]

cc: @holgerd77

[poojaranjan]

Suggested minor edits. Wondering if the upgrade process flow can be added for a quick visual of the phases and EIP status instructions.

[timbeiko]

@poojaranjan thanks for the quick review! Addressed the bulk of your comments in the last commit. Still two open threads to discuss.

Re: the diagram, if we have an updated one, yes, we should add it We can also add it in a subsequent PR if we don't have one now.

[poojaranjan]

@timbeiko
I'm fine with making the edits later on. Comments resolved.

Diagram: I have got the updated diagram here. One major change from the earlier version, CFI approved is moved to CFI phase. Looks like it was mistakenly shown in the Devnet phase. If you'd rather like to have only one CFI instead of applied and approved, I can make the edits, let me know.

[timbeiko]

Thanks @poojaranjan just added the diagram in a separate commit 😄 !