depends: Mention RISC-V known compilation issue with gcc-7.3.x

Author: laanwj

depends/README.md

@@ -49,6 +49,9 @@ For linux RISC-V 64-bit cross compilation (there are no packages for 32-bit):
 
     sudo apt-get install curl g++-riscv64-linux-gnu binutils-riscv64-linux-gnu
 
+RISC-V known issue: gcc-7.3.0 and gcc-7.3.1 result in a broken `test_bitcoin` executable (see https://github.com/bitcoin/bitcoin/pull/13543),
+this is apparently fixed in gcc-8.1.0.
+
 Dependency Options:
 The following can be set when running make: make FOO=bar