Add build instructions & Containerfile to compile the app

[emanuelb]

Add build instructions to README file.

1. create keystore.
2. generate genesisvision.properties file genesisvision.properties file missing #1 (comment)
3. run ./gradlew assembleRelease

Below Containerfile used to compile the app with all above steps:
Run podman build --pull --rm -t genesis_build_apk -f Containerfile

FROM frolvlad/alpine-glibc

RUN set -ex; \
    apk update; \
    apk add --no-cache \
        openjdk8 \
        git \
        bash; \
    adduser -D appuser;

USER appuser

ENV ANDROID_SDK_ROOT="/home/appuser/app/sdk" \
    ANDROID_HOME="/home/appuser/app/sdk"

RUN set -ex; \
    mkdir -p "/home/appuser/app/sdk/licenses" "/home/appuser/app/genesis/"; \
    printf "\n24333f8a63b6825ea9c5514f83c2829b004d1fee" > "/home/appuser/app/sdk/licenses/android-sdk-license"; \
    cd /home/appuser/app/genesis/; \
    git clone --depth 1 https://github.com/GenesisVision/android-client; \
    cd android-client; \
    keytool -genkey -alias genesis_releasekey -keystore /home/appuser/app/genesis/genesis.jks -storetype JKS -keyalg RSA -keysize 4096 -storepass genesis_releasekey -keypass genesis_releasekey -validity 10000 -dname CN=IL; \
printf 'tournamentApiAddress="localhost"
apiAddress="localhost"
webAddress="localhost"
devApiAddress="localhost"
devWebAddress="localhost"
binanceApiAddress="localhost"
binanceSocketAddress="localhost"
keystore=/home/appuser/app/genesis/genesis.jks
keystore.password=genesis_releasekey
keyAlias=genesis_releasekey
keyPassword=genesis_releasekey' > genesisvision.properties;

WORKDIR /home/appuser/app/genesis/android-client/

RUN ./gradlew assembleRelease