Used direct Zenodo link for Gnuastro download

The link describing where to get the necessary version of Gnuastro was
changed to point directly to the file in Zenodo. Also, the README file
was slightly edited.

README

@@ -2,10 +2,11 @@ Reproduction pipeline for Section 4 of "The MUSE Hubble Ultra Deep
 Field Survey: I. Survey description, data reduction and source
 detection", Bacon et al. (2017), A&A 608, A1.
 
-The purpose of this reproduction pipline is to show the calibration of
-pseudo-broad-band images created from MUSE data cubes with HST's
-broad-band images. For any issues with the pipeline, please contact
-Mohammad Akhlaghi at `mohammad@akhlaghi.org'.
+The purpose of this reproduction pipline is to show the photometric
+and astrometric calibration of pseudo-broad-band images created from
+the MUSE data cubes described in the paper with HST's broad-band
+images of the same field. For any issues with the pipeline, please
+contact Mohammad Akhlaghi at `mohammad@akhlaghi.org'.
 
 
 
@@ -70,10 +71,11 @@ difference.
   Gnuastro is a large collection of programs for astronomical data
   analysis on the command-line. This is an intermediate version of
   Gnuastro (the tarball is not officially released on the Gnuastro
-  webpage). But for convenience, this tarball is available in the
+  webpage). However, this pipeline will ONLY work with this version of
+  Gnuastro. But for convenience, this tarball is available in the
   following link:
 
-    https://doi.org/10.5281/zenodo.1163746
+    https://zenodo.org/record/1163746/files/gnuastro-0.2.51-bc56.tar.gz
 
   To uncompress, build and install the Gnuastro tarball, follow the
   instructions here:
@@ -89,8 +91,8 @@ difference.
     $ git checkout bc565a5
 
   Afterwords, you need to bootstrap Gnuastro as described in the
-  following link. Afterwards, you can configure and build it as in the
-  link above.
+  following link and configure and build it as described in the link
+  above.
 
     https://www.gnu.org/software/gnuastro/manual/html_node/Bootstrapping.html