Add Caveat: Readers need write access. Whitespace.

libraries/liblmdb/lmdb.h

@@ -66,6 +66,10 @@
  *	  BSD systems or when otherwise configured with MDB_USE_POSIX_SEM.
  *	  Multiple users can cause startup to fail later, as noted above.
  *
+ *	- There is normally no pure read-only mode, since readers need write
+ *	  access to locks and lock file. Exceptions: On read-only filesystems
+ *	  or with the #MDB_NOLOCK flag described under #mdb_env_open().
+ *
  *	- A thread can only use one transaction at a time, plus any child
  *	  transactions.  Each transaction belongs to one thread.  See below.
  *	  The #MDB_NOTLS flag changes this for read-only transactions.