A Raspberry Pi install page; the build's job count is the core count (from dev 6f49f498c3c6)

doc/install/linux_source_install.md

@@ -8,7 +8,9 @@ software.
 
 For the container installation on Linux, see `linux_container_install.md`.
 On Windows, this page also applies unchanged inside the Windows Subsystem
-for Linux; `windows_wsl_install.md` says what is specific to WSL.
+for Linux; `windows_wsl_install.md` says what is specific to WSL. On a
+Raspberry Pi it applies as well; `raspberry_pi_install.md` says what is
+specific to that machine.
 
 
 ## Prerequisites
@@ -109,6 +111,15 @@ you want to install and run:
 make -C com        # lib/libklammertext.so + sks/*.so + bin/{ktext,kdesc,kdiag}
 ```
 
+The build compiles as many files at once as the machine has cores (`-j`
+with the core count, in the variable `JOBS`), and more jobs than cores
+gain nothing: a compile keeps a core busy, so eight jobs on a four-core
+machine only take turns. On a machine with little memory the optimizing
+compiler may run out of it and report `out of memory allocating`; if so,
+`make -C com JOBS=2`, or `JOBS=1`: slower, and it finishes. A Raspberry Pi
+5 (four cores, 8 GB) builds with its four in under five minutes and needs
+no such limit.
+
 (For a slower `-O0` debug build with AddressSanitizer, intended for development,
 prefix `DEBUG=1`: `DEBUG=1 make -C com`.)