doc: Explain how to update chainTxData in release process

Adds a short explanation how to update chainTxData to the release
process. Mention where to get the data, and link to an example.

doc/release-process.md

@@ -23,7 +23,8 @@ Before every major release:
 
 * Update hardcoded [seeds](/contrib/seeds/README.md), see [this pull request](https://github.com/bitcoin/bitcoin/pull/7415) for an example.
 * Update [`BLOCK_CHAIN_SIZE`](/src/qt/intro.cpp) to the current size plus some overhead.
-* Update `src/chainparams.cpp` chainTxData with statistics about the transaction count and rate.
+* Update `src/chainparams.cpp` chainTxData with statistics about the transaction count and rate. Use the output of the RPC `getchaintxstats`, see
+  [this pull request](https://github.com/bitcoin/bitcoin/pull/12270) for an example.
 * Update version of `contrib/gitian-descriptors/*.yml`: usually one'd want to do this on master after branching off the release - but be sure to at least do it before a new major release
 
 ### First time / New builders