Docs: move CARGO_BUILD_TARGET note

I think it makes more sense to put it amidst the other instructions,
rather than keep it at the very end.

README.md

@@ -95,6 +95,9 @@ There are numerous ways:
     sure to install the header files as well (on Debian and derivatives, headers
     are in `-dev` packages, e.g. `libsqlite3-dev`.)
 
+    Cross-compilers need to set `CARGO_BUILD_TARGET`; see [cargo
+    documentation](https://doc.rust-lang.org/cargo/reference/config.html#environment-variables).
+
     Then compile and install with:
 
       $ make                   #  pass -jN to use N CPU cores, e.g. -j8
@@ -105,9 +108,6 @@ There are numerous ways:
 
     To uninstall, run `sudo make uninstall`.
 
-Cross-compilers need to set `CARGO_BUILD_TARGET`; see [cargo
-documentation](https://doc.rust-lang.org/cargo/reference/config.html#environment-variables).
-
 <!--
     UPDATE doc/newsboat.asciidoc IF YOU CHANGE THIS LIST
 -->

doc/newsboat.asciidoc

@@ -115,14 +115,13 @@ There are a few different ways:
   $ sudo make install      #  install everything under /usr/local
 +
 To install to a different directory, pass `prefix` like so: `sudo make
-prefix=/opt/newsboat install`.
+prefix=/opt/newsboat install`. If you're cross-compiling, set
+`CARGO_BUILD_TARGET`; see
+https://doc.rust-lang.org/cargo/reference/config.html#environment-variables[cargo
+documentation] for details.
 +
 To uninstall, run `sudo make uninstall`.
 
-Cross-compilers need to set `CARGO_BUILD_TARGET`; see
-https://doc.rust-lang.org/cargo/reference/config.html#environment-variables[cargo
-documentation].
-
 // UPDATE README.md IF YOU CHANGE THIS LIST
 
 == First Steps