• Add upgrades.json to the root of the repo with all of the upgrade information in it, including epochs, dates and actors CIDs for mainnet and calibrationnet. This can be consumed programatically and be used tasks such as informing users that upgrades are pending. Suggested by @magik6k.
• Add an _upgrades.js executable in the Network Upgrades directory that will generate the README.md in that directory from the upgrades.json data.

In the process a few minor inconsistencies were fixed up, including one FIP reference that pointed to a pull request.

Compare rendered versions side by side for accuracy:

• https://github.com/filecoin-project/core-devs/blob/master/Network%20Upgrades/README.md
• https://github.com/rvagg/core-devs/blob/rvagg/upgrades.json/Network%20Upgrades/README.md

As per docs inside _upgrades.js, here's some quirks about the data that may be relevant to consumers:

// Some quirks about the upgrades.json file:
// - If the activation.mainnet.epoch field is null, it means the epoch is TBD.
// - If the activation.mainnet.epoch field is null but the activation.mainnet.time field is not null,
//   it means the epoch is TBD but the date and time have been proposed.
// - builtin-actors began at version 8, so we don't link to the actors repo for upgrades before that.

In addition, an upgrade with an actors version >8 without actors CIDs means that it hasn't been tagged.

/cc @rjan90 @BigLep