Add mention of cargo requirement for rust frontend

Cargo is temporarily required in order to build some parts of GCC rust
frontend. This requirement was not mentioned in the documentation.

gcc/ChangeLog:

	* doc/install.texi: Mention cargo requirement.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

Author: P-E-P

gcc/doc/install.texi

@@ -313,6 +313,13 @@ documentation including the target @code{SYSTEM} definition module.
 If Python3 is unavailable Modula-2 documentation will include a target
 independent version of the SYSTEM modules.
 
+@item @anchor{GCCRS-prerequisite}Rust
+
+In order to build GCCRS you need a working rust compiler, for now we
+are using cargo and rustc but in the future GCC will be able to compile
+it's own rust components and recompile itself with those components
+included.
+
 @item A ``working'' POSIX compatible shell, or GNU bash
 
 Necessary when running @command{configure} because some
@@ -3227,6 +3234,15 @@ and network filesystems.
 @uref{prerequisites.html#GDC-prerequisite,,GDC prerequisites}.
 @end ifhtml
 
+@section Building the Rust compiler
+
+@ifnothtml
+@ref{GCCRS-prerequisite}.
+@end ifnothtml
+@ifhtml
+@uref{prerequisites.html#GCCRS-prerequisite,,GCCRS prerequisites}.
+@end ifhtml
+
 @section Building with profile feedback
 
 It is possible to use profile feedback to optimize the compiler itself.  This