Merge #14882: [doc] developer-notes.md: point out that UniValue deviates from upstream

a67d71311d [doc] developer-notes.md: point out that UniValue deviates from upstream (Sjors Provoost)

Pull request description:

  While debugging an issue I was somewhat surprised to [learn](https://github.com/bitcoin/bitcoin/pull/14164#issuecomment-419752678) that we've moved `src/univalue` from https://github.com/jgarzik/univalue to https://github.com/bitcoin-core/univalue, that these repos are both maintained and they're different.

  The first mention of using the bitcoin-core repo is from late 2015 in #7157. I didn't check when the last common ancestor commit is.

  I couldn't find documentation as to why (these things just happen in open source of course), but at minimum we should make this more clear.

  There's also the following line in `config.ac` that I'm not sure what to do with:
  ```
  AC_INIT([univalue], [1.0.3],
          [http://github.com/jgarzik/univalue/])
  ```

Tree-SHA512: e58105677b5ebe0005772282da4a805fee7dfccacfb1b2686a874517bf46072d1481181f8a8865d25526f6ed9e5fcd55d8d49906bf27cd0f5aefe4f258aa4d63

doc/developer-notes.md

@@ -741,7 +741,7 @@ Current subtrees include:
   - Upstream at https://github.com/bitcoin-core/ctaes ; actively maintained by Core contributors.
 
 - src/univalue
-  - Upstream at https://github.com/jgarzik/univalue ; report important PRs to Core to avoid delay.
+  - Upstream at https://github.com/bitcoin-core/univalue ; actively maintained by Core contributors, deviates from upstream https://github.com/jgarzik/univalue
 
 Upgrading LevelDB
 ---------------------