Added documentation for Install application from APK (#31)

Co-authored-by: Priyansh Garg <priyanshgarg30@gmail.com>

docs/install-android-application-from-apk.md

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+# Installing an application from APK
+
+## Introduction
+
+This guide will show you how to install an application from an APK file on an Android emulator or a real device.
+
+## Prerequisites
+
+- Setup Android Emulator using [mobile-helper-tool](https://github.com/nightwatchjs/mobile-helper-tool): `npx @nightwatch/mobile-helper android`.
+- APK file of the application you want to install
+
+### Android Device Bridge (adb)
+
+The whole process is done using `adb` command line tool. So, you would need to make sure that `adb` is available from your terminal. You can check this by running `adb --version`.
+
+If `adb` is not available directly, you can either add its location to your `PATH` environment variable, or `cd` to the location where the `adb` binary is present and use it directly from there.
+
+For both the cases, you'd need the location where you've setup your Android SDK, which you can get that by running `npx @nightwatch/mobile-helper android` again:
+
+![Alt text](image.png)
+
+The `adb` binary will be present in the `platform-tools` sub-directory of your Android SDK setup location from above. Eg: `/path/to/Android/sdk/platform-tools/`.
+
+### Adding `adb` location to `PATH`
+
+Add the path of `platform-tools` directory to your `PATH` environment variable.
+
+**Linux/Mac**:
+
+Add the below command at the end of your `~/.bashrc` or `~./bash_profile` file and restart the terminal.
+
+```bash
+export PATH=$PATH:/path/to/Android/sdk/platform-tools/
+```
+
+**Windows**:
+
+Add the below path to your `PATH` environment variable in the Control Panel and restart the terminal.
+
+```bash
+\path\to\Android\sdk\platform-tools\
+```
+
+### Using `adb` directly
+
+To use `adb` directly (without adding it to `PATH`), simply go to the directly where the binary is present and use it as follows:
+
+```bash
+cd /path/to/Android/sdk/platform-tools/
+
+# for windows
+adb.exe --version
+
+# for mac/linux
+./adb --version
+```
+
+## Assumptions
+
+- APK path is `/path/to/your/app.apk`
+- Application package name is `your.app.package`
+
+## Steps
+
+### Check if device is connected
+
+```bash
+adb devices
+```
+
+### Install the application
+
+```bash
+adb install /path/to/your/app.apk
+```
+
+### Verify the installation
+
+```bash
+adb shell pm list packages -f your.app.package
+```
+
+**Note:** If the don't know the exact name of your package, you can use `grep` to find it from the list of installed packages. `grep` comes pre-installed on Linux and Mac terminals.
+
+```bash
+adb shell pm list packages -f | grep package_name
+```
+
+### Overwrite the existing application
+
+If you want to overwrite the existing application with the new APK, you can use the `-r` option with the `install` command.
+
+```bash
+adb install -r /path/to/your/app.apk
+```
+
+### Remove the application
+
+```bash
+adb uninstall your.app.package
+```
+
+### Install the application on a specific device
+
+If you have multiple devices/emulators connected to your adb, you will get an error when you try to install the application.
+You need to specify the device on which you want to install the application.
+
+This is not just for installation, you need to specify the device for any `adb` command you run.
+
+To install the application on a specific device, you need to specify the device using the `-s` option before the `install` command.
+
+```bash
+adb -s <device_id> install /path/to/your/app.apk
+```
+
+You can get the device id by running the below command.
+
+```bash
+adb devices
+```
+
+```bash
+# Output of adb devices
+List of devices attached
+emulator-5554	device
+emulator-5556	device
+```
+
+Here, `emulator-5554` and `emulator-5556` are the device ids.