Update README.md to point at Libera.chat

[crabbedhaloablution]

##openfpga on freenode /topic:
Topic for ##openfpga is "Moved to another server. Please ask here for details."
Topic set by balrog on 2021-05-26 03:28:29 UTC

##openfpga on Libera.chat /topic:
Topic for ##openfpga is "Open source tools for FPGAs, CPLDs, etc. Silicon RE, bitfile RE, synthesis, place-and-route, and JTAG are all on topic. Code: https://github.com/azonenberg/openfpga. Channel logs: https://libera.irclog.whitequark.org/##openfpga"
Topic set by azonenberg!~azonenber@gw-ipv4.poulsbo.antikernel.net on 2021-05-23 22:56:12 UTC