README: Add OpenOCD to dependencies

Signed-off-by: David Shah <dave@ds0.me>

README.md

diff --git a/README.md b/README.md
index 4af0768..50455a8 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,8 @@
  - CMake 3.5 or later
  - Boost including boost-python
  - Git
- 
+ - Recent OpenOCD for device programming (`--enable-ftdi` required if building from source)
+
 Clone the Project Trellis repository and download the latest database:
 
      git clone --recursive https://github.com/SymbiFlow/prjtrellis