docs: replace dead original whitepaper link (#4155)

Co-Authored-By: Marko <marbar3778@yahoo.com>

CHANGELOG_PENDING.md

@@ -68,6 +68,7 @@ program](https://hackerone.com/tendermint).
 - [crypto] Add `RegisterKeyType` to amino to allow external key types registration (@austinabell)
 - [rpc] \#3188 Added `block_size` to `BlockMeta` this is reflected in `/blockchain`
 - [types] \#2521 Add `NumTxs` to `BlockMeta` and `EventDataNewBlockHeader`
+- [docs] [\#4111](https://github.com/tendermint/tendermint/issues/4111) Replaced dead whitepaper link in README.md
 
 ### BUG FIXES:
 

README.md

@@ -157,5 +157,6 @@ Additional documentation is found [here](/docs/tools).
 
 - [The latest gossip on BFT consensus](https://arxiv.org/abs/1807.04938)
 - [Master's Thesis on Tendermint](https://atrium.lib.uoguelph.ca/xmlui/handle/10214/9769)
-- [Original Whitepaper](https://tendermint.com/static/docs/tendermint.pdf)
+- [Original Whitepaper](https://github.com/tendermint/spec)
+   - You can find the link at the bottom of the readme
 - [Blog](https://blog.cosmos.network/tendermint/home)