update README.md with link to nextpnr ECP5 add link to https://github.com/YosysHQ/nextpnr#nextpnr-ecp5

diff --git a/README.md b/README.md
index 39bbb65..a738631 100644
--- a/README.md
+++ b/README.md

@@ -30,7 +30,7 @@
 
 Clone and install **latest git master** versions (Yosys 0.8 is not sufficient for ECP5 development) of [Yosys](https://github.com/YosysHQ/yosys)
  and [nextpnr](https://github.com/YosysHQ/nextpnr) according to their own instructions. Ensure
- to include the ECP5 architecture when building nextpnr; and point it towards your prjtrellis
+ to include the [ECP5 architecture](https://github.com/YosysHQ/nextpnr#nextpnr-ecp5) when building nextpnr; and point it towards your prjtrellis
  folder.  (for example: `cmake -DARCH=ecp5 -DTRELLIS_INSTALL_PREFIX=/usr/share/trellis/ .`)
 
 You should now be able to build the [examples](examples).