added haoyu paper on removed apps from google play

android.bib

@@ -708,3 +708,14 @@
   crossref =	 "ICPC2018",
   month =	 may,
 }
+
+@inproceedings{Wang:2018:WAA,
+ author = {Wang, Haoyu and Li, Hao and Li, Li and Guo, Yao and Xu, Guoai},
+ title = {Why Are Android Apps Removed from Google Play?: A Large-scale Empirical Study},
+ crossref = "MSR2018",
+ isbn = {978-1-4503-5716-6},
+ pages = {231--242},
+ doi = {10.1145/3196398.3196412},
+ publisher = {ACM},
+ keywords = {Android, app mining, app store, malware},
+}