board/visionfive2: add link to documentation

Add a link to RVspace Documentation Center, which did not exist
when readme.txt was first submitted. It provides datasheet, quick
start, schematics, and so on.

Signed-off-by: Francois Dugast <francois.dugast.foss@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/visionfive2/readme.txt

@@ -4,6 +4,8 @@ Starfive VisionFive2
 The VisionFive2 is a low-cost RISC-V 64-bit based platform, powered by a
 Starfive JH7110 processor.
 
+https://doc-en.rvspace.org/Doc_Center/visionfive_2.html
+
 How to build
 ============