mobile: improved documentation for building Android [skip ci]

docs/release-notes/release-notes-0.15.0.md

@@ -58,4 +58,5 @@
 * Joost Jager
 * Liviu
 * Torkel Rogstad
+* Vsevolod Kaganovych
 * Yong Yu

mobile/README.md

@@ -2,6 +2,18 @@
 
 ## Prerequisites
 
+### Go language
+
+- [Install](https://github.com/lightningnetwork/lnd/blob/master/docs/INSTALL.md#preliminaries-for-installing-from-source) `GO` language.
+- Provide `$GOPATH` to `.zshrc` or `.bash_profile` files.
+
+```shell
+export GOPATH=$HOME/go
+export PATH=$PATH:$GOPATH/bin
+```
+
+Or any path you want it to be in.
+
 ### Docker
 
 - Install and run [Docker](https://www.docker.com/products/docker-desktop).
@@ -16,17 +28,21 @@
 
 ### Go mobile
 
-Install [gomobile](https://github.com/golang/go/wiki/Mobile) and initialize it:
+- Install [gomobile](https://github.com/golang/go/wiki/Mobile):
 
 ```shell
 ⛰  go install golang.org/x/mobile/cmd/gomobile@latest
-⛰  gomobile init
+```
+
+- Install `gobind`
+
+```shell
+⛰  go install golang.org/x/mobile/cmd/gobind@latest
 ```
 
 ## Building the libraries
 
-Note that `gomobile` only supports building projects from `GOPATH` at this
-point. So, before continuing, be sure to be in the `src` folder:
+Note that `gomobile` only supports building projects from `GOPATH` at this point. So, before continuing, be sure to be in the `src` folder:
 
 ```shell
 ⛰  cd $GOPATH/src/github.com/lightningnetwork/lnd
@@ -38,6 +54,26 @@ To checkout the latest tagged release of lnd, run
 ⛰  git checkout $(git describe --tags --abbrev=0)
 ```
 
+Or, the second option: just clone the project and checkout to any branch/tag (pay attention to the dot in the end).
+
+```shell
+⛰  git clone https://github.com/lightningnetwork/lnd .
+```
+
+#### For Android:
+
+Move to the folder or create one:
+
+```shell
+⛰  cd $GOPATH/src/golang.org/x
+```
+
+After that clone the goland mobile repo
+
+```shell
+⛰  git clone https://github.com/golang/mobile
+```
+
 ### Building `lnd` for iOS
 
 ```shell
@@ -46,6 +82,8 @@ To checkout the latest tagged release of lnd, run
 
 ### Building `lnd` for Android
 
+Go to `$GOPATH/src/github.com/lightningnetwork/lnd` and run the command below (make sure that the Docker  engine is running):
+
 ```shell
 ⛰  make android
 ```
@@ -114,22 +152,99 @@ to your project to support the generated code.
 
 ### Android
 
+#### First option:
+
 In order to generate protobuf definitions for Android, add `--java_out=.`
 
 to the first `protoc` invocation found in
 [ `gen_protos.sh` ](../lnrpc/gen_protos.sh). Then, run `make rpc`.
 
+
+#### Second option (preferable):
+
+- You have to install the profobuf plugin to your Android application. 
+Please, follow this link https://github.com/google/protobuf-gradle-plugin.
+- Add this line to your `app build.gradle` file.
+```shell
+classpath "com.google.protobuf:protobuf-gradle-plugin:0.8.17"
+```
+- Create a `proto` folder under the `main` folder.
+
+![proto_folder](docs/proto_folder.png)
+
+- Add `aar` file to libs folder.
+
+- After that add these lines to your `module's` `build.gradle` file:
+
+```shell
+plugins {
+    id "com.google.protobuf"
+}
+
+android {
+    sourceSets {
+        main {
+            proto {
+
+            }
+        }
+    }
+}
+
+dependencies {
+    implementation fileTree(dir: "libs", include: ["*.jar"])
+    implementation "com.google.protobuf:protobuf-javalite:${rootProject.ext.javalite_version}"
+}
+
+protobuf {
+    protoc {
+        artifact = "com.google.protobuf:protoc:${rootProject.ext.protoc_version}"
+    }
+    generateProtoTasks {
+        all().each { task ->
+            task.builtins {
+                java {
+                    option "lite"
+                }
+            }
+        }
+    }
+}
+```
+- Then, copy all the proto files from `lnd/lnrpc` to your `proto` folder, saving the structure.
+- Build the project and wait until you see the generated Java proto files in the `build` folder.
+
+#### Note:
+
+If Android Studio tells you that the `aar` file cannot be included into the `app-bundle`, this is a workaround:
+
+1. Create a separate gradle module
+2. Remove everything from there and leave only `aar` and `build.gradle`.
+
+![separate_gradle_module](docs/separate_gradle_module.png)
+
+3. Gradle file should countain only these lines:
+
+```shell
+configurations.maybeCreate("default")
+artifacts.add("default", file('Lndmobile.aar'))
+```
+
+4. In `dependencies` add this line instead of depending on `libs` folder:
+```shell
+implementation project(":lndmobile", { "default" })
+```
+
+
 ## Options
 
-Similar to lnd, subservers can be conditionally compiled with the build by
-setting the tags argument:
+Similar to lnd, subservers can be conditionally compiled with the build by setting the tags argument:
 
 ```shell
 ⛰  make ios
 ```
 
-To support subservers that have APIs with name conflicts, pass the "prefix"
-flag. This will add the subserver name as a prefix to each method name:
+To support subservers that have APIs with name conflicts, pass the "prefix" flag. This will add the subserver name as a prefix to each method name:
 
 ```shell
 ⛰  make ios prefix=1