[docs] Document src subdirectories and different libraries

Author: jnewbery

src/Makefile.am

@@ -245,6 +245,9 @@ obj/build.h: FORCE
 libbitcoin_util_a-clientversion.$(OBJEXT): obj/build.h
 
 # server: shared between bitcoind and bitcoin-qt
+# Contains code accessing mempool and chain state that is meant to be separated
+# from wallet and gui code (see node/README.md). Shared code should go in
+# libbitcoin_common or libbitcoin_util libraries, instead.
 libbitcoin_server_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(MINIUPNPC_CPPFLAGS) $(EVENT_CFLAGS) $(EVENT_PTHREADS_CFLAGS)
 libbitcoin_server_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 libbitcoin_server_a_SOURCES = \

src/node/README.md

@@ -0,0 +1,22 @@
+# src/node/
+
+The [`src/node/`](./) directory contains code that needs to access node state
+(state in `CChain`, `CBlockIndex`, `CCoinsView`, `CTxMemPool`, and similar
+classes).
+
+Code in [`src/node/`](./) is meant to be segregated from code in
+[`src/wallet/`](../wallet/) and [`src/qt/`](../qt/), to ensure wallet and GUI
+code changes don't interfere with node operation, to allow wallet and GUI code
+to run in separate processes, and to perhaps eventually allow wallet and GUI
+code to be maintained in separate source repositories.
+
+As a rule of thumb, code in one of the [`src/node/`](./),
+[`src/wallet/`](../wallet/), or [`src/qt/`](../qt/) directories should avoid
+calling code in the other directories directly, and only invoke it indirectly
+through the more limited [`src/interfaces/`](../interfaces/) classes.
+
+The [`src/node/`](./) directory is a new directory introduced in
+[#14978](https://github.com/bitcoin/bitcoin/pull/14978) and at the moment is
+sparsely populated. Eventually more substantial files like
+[`src/validation.cpp`](../validation.cpp) and
+[`src/txmempool.cpp`](../txmempool.cpp) might be moved there.