riscv: add README for patchset release

Author: IgnotaYun

README.md

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+# LJRV - LuaJIT RISC-V 64 Port
+
+LuaJIT is a Just-In-Time (JIT) compiler for the Lua programming language,
+RISC-V is a free and open ISA enabling a new era of processor innovation.
+
+Find latest LJRV release at [IgnotaYun/LuaJIT,v2.1-riscv64](https://github.com/IgnotaYun/LuaJIT/tree/v2.1-riscv64).
+Development branch is avaliable at [IgnotaYun/LuaJIT,riscv](https://github.com/IgnotaYun/LuaJIT/tree/riscv).
+
+**This is the release branch of LJRV, containing patchset based on dev branch commit [1ce02ddf8b6e ("riscv(interp): fix cdata matching handler in BC_ISEQV & BC_ISNEV")](https://github.com/IgnotaYun/LuaJIT/commit/1ce02ddf8b6e705f7161a8e88dd6c42884224d5c), based off LuaJIT v2.1 rolling [194d7f2d635a ("Merge branch 'master' into v2.1")](https://github.com/LuaJIT/LuaJIT/commit/194d7f2d635a11193177f0ed820ae419148f0b70)**
+
+## Introduction
+
+LJRV is a ongoing porting project of LuaJIT to the RISC-V 64-bit architecture by ISRC, ISCAS.
+The ultimate goal is to provide a RISC-V 64 LuaJIT implementation and have it upstreamed to the official LuaJIT repository.
+
+## Building and Packaging
+
+LJRV is built and packaged in the same way as LuaJIT, requires a recent toolchain based on GCC or Clang with RISC-V 64 support and GNU Make.
+For building and installation instructions, please refer to the [LuaJIT README](https://luajit.org/install.html).
+
+For distro maintainers and packagers, we encourage one to follow our branch and patchset, as we would keep it up-to-date with the latest LuaJIT upstream. Cherry-picking and backporting is **strongly** discouraged, ["no matter how self-standing individual changes look (because they often are not)"](https://luajit.org/download.html).
+
+## Progress
+
+- [x] Interpreter Runtime
+- [x] JIT Compiler
+
+LJRV is still considered of beta quality, take it with a grain of salt.
+
+FFI struct passing is known to be partially broken, please report any issue you encounter.
+
+## Bug Report
+
+Please report bugs to [Issues](https://github.com/ruyisdk/LuaJIT/issues).
+
+## Copyright
+
+LuaJIT is Copyright (C) 2005-2026 Mike Pall.
+LuaJIT is free software, released under the MIT license.
+See full Copyright Notice in the COPYRIGHT file or in luajit.h.
+
+LJRV is Copyright (C) 2022-2026 ISRC, ISCAS. Contributed by gns.
+LJRV is free software, released under the MIT license.
+LJRV is part of openRuyi.