Add docs for dependent transactions.

This commit adds documentation to clarify that read-only and read-write
transactions should not be mixed in the same goroutine as it can cause
deadlocks during remapping.

See: https://github.com/boltdb/bolt/issues/378

README.md

@@ -87,6 +87,11 @@ are not thread safe. To work with data in multiple goroutines you must start
 a transaction for each one or use locking to ensure only one goroutine accesses
 a transaction at a time. Creating transaction from the `DB` is thread safe.
 
+Read-only transactions and read-write transactions should not depend on one
+another and generally shouldn't be opened simultaneously in the same goroutine.
+This can cause a deadlock as the read-write transaction needs to periodically
+re-map the data file but it cannot do so while a read-only transaction is open.
+
 
 #### Read-write transactions
 

db.go

@@ -401,6 +401,11 @@ func (db *DB) close() error {
 // will cause the calls to block and be serialized until the current write
 // transaction finishes.
 //
+// Transactions should not be depedent on one another. Opening a read
+// transaction and a write transaction in the same goroutine can cause the
+// writer to deadlock because the database periodically needs to re-mmap itself
+// as it grows and it cannot do that while a read transaction is open.
+//
 // IMPORTANT: You must close read-only transactions after you are finished or
 // else the database will not reclaim old pages.
 func (db *DB) Begin(writable bool) (*Tx, error) {