Rename MJIT filenames to RJIT

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+# MJIT
+
+This document has some tips that might be useful when you work on MJIT.
+
+## Supported platforms
+
+The following platforms are either tested on CI or assumed to work.
+
+* OS: Linux, macOS
+* Arch: x86\_64, aarch64, arm64, i686, i386
+
+### Not supported
+
+The MJIT support for the following platforms is no longer maintained.
+
+* OS: Windows (mswin, MinGW), Solaris
+* Arch: SPARC, s390x
+
+## Developing MJIT
+
+### Bindgen
+
+If you see an "MJIT bindgen" GitHub Actions failure, please commit the `git diff` shown on the failed job.
+
+For doing the same thing locally, run `make mjit-bindgen` after installing libclang.
+macOS seems to have libclang by default. On Ubuntu, you can install it with `apt install libclang1`.
+
+### Always run make install
+
+Always run `make install` before running MJIT. It could easily cause a SEGV if you don't.
+MJIT looks for the installed header for security reasons.
+
+### --mjit-debug vs --mjit-debug=-ggdb3
+
+`--mjit-debug=[flags]` allows you to specify arbitrary flags while keeping other compiler flags like `-O3`,
+which is useful for profiling benchmarks.
+
+`--mjit-debug` alone, on the other hand, disables `-O3` and adds debug flags.
+If you're debugging MJIT, what you need to use is not `--mjit-debug=-ggdb3` but `--mjit-debug`.