Add Play Store badge

README.md

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 NASA Space Apps Challenge 2018 product.
 
+<a href='https://play.google.com/store/apps/details?id=com.exos.escapeearth'><img alt='Get it on Google Play' src='https://play.google.com/intl/en_us/badges/images/generic/en_badge_web_generic.png' height="80"/></a>
+
 ## The challenge
 
 **Do YOU Know When the Next Rocket Launch Is?**
@@ -57,3 +59,5 @@ PowerPoint presentation: https://drive.google.com/uc?id=1RUj_obKkk_qJa7SKBvSx8Ys
 
  You should have received a copy of the GNU General Public License
  along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+_Google Play and the Google Play logo are trademarks of Google LLC._