gitian: Fix README inclusion in archives

Linux:
The README was originally added in 8550f1f, but included the README
under the docs directory, which has a bunch of internal links that won't
make sense in a release tarball. In this patch, we include the root
level README instead, which makes more sense.

Windows:
.md files are inconvenient to open on windows and the line endings
differ, so we use README_windows.txt instead.

contrib/gitian-descriptors/gitian-linux.yml

@@ -181,7 +181,7 @@ script: |
     rm -rf ${DISTNAME}/lib/pkgconfig
     find ${DISTNAME}/bin -type f -executable -print0 | xargs -0 -n1 -I{} ../contrib/devtools/split-debug.sh {} {} {}.dbg
     find ${DISTNAME}/lib -type f -print0 | xargs -0 -n1 -I{} ../contrib/devtools/split-debug.sh {} {} {}.dbg
-    cp ../doc/README.md ${DISTNAME}/
+    cp ../README.md ${DISTNAME}/
     find ${DISTNAME} -not -name "*.dbg" | sort | tar --mtime="$REFERENCE_DATETIME" --no-recursion --mode='u+rw,go+r-w,a+X' --owner=0 --group=0 -c -T - | gzip -9n > ${OUTDIR}/${DISTNAME}-${i}.tar.gz
     find ${DISTNAME} -name "*.dbg" | sort | tar --mtime="$REFERENCE_DATETIME" --no-recursion --mode='u+rw,go+r-w,a+X' --owner=0 --group=0 -c -T - | gzip -9n > ${OUTDIR}/${DISTNAME}-${i}-debug.tar.gz
     cd ../../

contrib/gitian-descriptors/gitian-win.yml

@@ -166,6 +166,7 @@ script: |
     rm -rf ${DISTNAME}/lib/pkgconfig
     find ${DISTNAME}/bin -type f -executable -print0 | xargs -0 -n1 -I{} ../contrib/devtools/split-debug.sh {} {} {}.dbg
     find ${DISTNAME}/lib -type f -print0 | xargs -0 -n1 -I{} ../contrib/devtools/split-debug.sh {} {} {}.dbg
+    cp ../doc/README_windows.txt ${DISTNAME}/readme.txt
     find ${DISTNAME} -not -name "*.dbg"  -type f | sort | zip -X@ ${OUTDIR}/${DISTNAME}-${i}.zip
     find ${DISTNAME} -name "*.dbg"  -type f | sort | zip -X@ ${OUTDIR}/${DISTNAME}-${i}-debug.zip
     cd ../../