message: Added a note in QFAST docs regarding the caching
diff: @@ -1811,6 +1811,9 @@ Optionally, we can provide additional configurations as listed below.
  | initial-depth | The initial number of circuit layers. | int (default = 1) |
  +------------------------+------------------------------------------------------------------------+--------------------------------------+
  +By default, after each unitary matrix decomposition, the QFAST plugin will save the result (in terms of smaller block matrices) into a cache file located at ``$XACC_INSTALL_DIR/tmp``. If a cache entry is found, the QFAST plugin will re-use the result automatically.
  +
  +Users can modify the cache filename via the configuration key ``cache-file-name``.
  Example:
  