README.md: Add information about selecting different toolchain

Signed-off-by: Robert Winkler <rwinkler@antmicro.com>

README.md

@@ -211,9 +211,9 @@ Built-in commands:
 | FPGA family       |      Toolchain        |
 |-------------------|-----------------------|
 | Xilinx Ultrascale |      Vivado           |
-| Xilinx 7-Series   |      Vivado           |
+| Xilinx 7-Series   |   Vivado/SymbiFlow*   |
 | Xilinx Spartan6   |        ISE            |
-| Lattice ECP5      | Yosys/Trellis/Nextpnr |
+| Lattice ECP5      | Yosys+Trellis+Nextpnr |
 | Altera Cyclone4   |    Quartus Prime      |
 
 Once installed, build the bitstream with:
@@ -227,6 +227,12 @@ To load the bitstream to you board, run:
 $ ./make.py --board=XXYY --load
 ```
 > **Note**: If you are using a Versa board, you will need to change J50 to bypass the iSPclock. Re-arrange the jumpers to connect pins 1-2 and 3-5 (leaving one jumper spare). See p19 of the Versa Board user guide.
+
+> **Note:** \*=to select a different toolchain use the `--toolchain` option, i.e.:
+> ```
+> ./make.py --board=arty --toolchain=symbiflow --build
+> ```
+
 ### Load the Linux images over Serial
 All the boards support Serial loading of the Linux images and this is the only way to load them when the board does not have others communications interfaces or storage capability.