Update README.md and little hint to download (#191)

* refactor: add tip if DATADIR missing

* doc: tune README.md for new verison

README.md

@@ -10,4 +10,4 @@
 1. `git clone [repo] && cd AndroidLibXrayLite`
 2. `gomobile init`
 3. `go mod tidy -v`
-4. `gomobile bind -v -androidapi 21 -ldflags='-s -w' ./`
+4. `gomobile bind -v -androidapi 24 -trimpath -ldflags='-s -w -buildid= -checklinkname=0' ./`

gen_assets.sh

@@ -21,6 +21,11 @@ check_dependencies() {
 
 # Download data function
 download_dat() {
+    if [[ ! -d "$DATADIR" ]]; then
+        echo "Downloading failed \"$DATADIR\" does not exists"
+        exit 1
+    fi
+
     echo "Downloading geoip.dat..."
     curl -sL https://github.com/Loyalsoldier/v2ray-rules-dat/releases/latest/download/geoip.dat -o "$DATADIR/geoip.dat"