doc: Include a download link near the top of README.md

Signed-off-by: Michael Brown <[email protected]>

README.md

@@ -3,6 +3,13 @@ CX: Privacy-compatible distributed contact tracing
 
 ![Build documentation](https://github.com/ipxe/cx/workflows/Build%20documentation/badge.svg)
 
+Download the full [architecture
+specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+[![PDF](pdf.png)](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+
+Overview
+--------
+
 CX is an architecture for privacy-compatible distributed contact
 tracing, designed for use in pandemics such as Covid-19. It operates
 using a model of proximity-based contact detection over one or more
@@ -225,4 +232,5 @@ Details
 -------
 
 For more information, see the [architecture
-specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf).
+specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+[![PDF](pdf.png)](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)

spec/Makefile

@@ -54,10 +54,13 @@ cx-intro.md : cx.md Makefile
 ../%.png : %.svg Makefile
 	rsvg-convert -f png -d 150 -o $@ $<
 
+../pdf.png : clipart/pdf.svg Makefile
+	rsvg-convert -f png -h 32 -o $@ $<
+
 ../README.md : README-header.md.part cx-intro.md README-footer.md.part \
 	       ../broadcasting.png ../observing.png ../diagnosis.png \
 	       ../publication.png ../international.png ../cooperation.png \
-	       Makefile
+	       ../pdf.png Makefile
 	cat README-header.md.part cx-intro.md README-footer.md.part > $@
 
 readme : ../README.md Makefile

spec/README-footer.md.part

@@ -2,4 +2,5 @@ Details
 -------
 
 For more information, see the [architecture
-specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf).
+specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+[![PDF](pdf.png)](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)

spec/README-header.md.part

@@ -2,3 +2,10 @@ CX: Privacy-compatible distributed contact tracing
 ==================================================
 
 ![Build documentation](https://github.com/ipxe/cx/workflows/Build%20documentation/badge.svg)
+
+Download the full [architecture
+specification](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+[![PDF](pdf.png)](https://github.com/ipxe/cx/releases/latest/download/cx.pdf)
+
+Overview
+--------