Create Network upgrade postmortem template.md

[poojaranjan]

Network upgrade postmortem report template.

When opening a pull request to submit a new EIP, please use the suggested template: https://github.com/ethereum/EIPs/blob/master/eip-template.md

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[MicahZoltu]

Same comments as over in #2809, I don't think the EIP repository is the right place for post mortems of Ethereum mainnet operations.

[poojaranjan]

This PR is created to add a template to create the network upgrade postmortem report and be saved as an Informational EIP for community reference. 

Being a template, I've added Type = Meta (Ref. EIP 233, a template for creating the hardfork meta EIP). However, this is not a proposal related to the network standard, so, if suggested, "Type' can be changed to "Informational EIP" to get it merged. 

IMO, changing the type of proposal will let this template sit as a recommendation and not a mandate to be followed to create the network upgrade postmortem/assessment report. 

[lightclient]

I think this should be informational.

[lightclient]

Sorry, I take that back. I think this EIP should be meta, but it should specify that EIPs that follow this template are informational.

[github-actions]

There has been no activity on this pull request for two months. It will be closed in a week if no further activity occurs. If you would like to move this EIP forward, please respond to any outstanding feedback or add a comment indicating that you have addressed all required feedback and are ready for a review.

[github-actions]

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