[readme] Add multiplatform support description into readme.

GRADLE_PLUGIN.md

@@ -218,6 +218,41 @@ and set dependencies between building tasks.
     }
     ```
 
+## Multiplatform build
+
+Kotlin/Native, as well as Kotlin/JVM and Kotlin/JS, supports multiplatform projects. Such a support is included in the
+Kotlin/Native Gradle plugin by default and there is no need to apply additional plugins to use it. By default
+multiplatform support is turned off, and could be enabled with the `enableMultiplatform` DSL method:
+
+    apply 'konan'
+    
+    konanArtifacts {
+        program('foo') {
+            enableMultiplatform true
+        }
+    }
+
+The Gradle plugin adds an `expectedBy` dependency configuration that is used to specify a dependency from Kotlin/Native
+project to a common project:
+
+    apply 'konan'
+    
+    dependencies {
+        expectedBy project('commonProject')
+    }
+
+When a common project is added as an `expectedBy` dependency, all the artifacts with the multiplatform support enabled
+will use it's `main` source set as a common module. One may specify a custom source set for each artifact using the
+`commonSourceSet` DSL method. In this case the multiplatform support will be also enabled for this artifact.
+
+    konanArtifacts {
+        program('foo') {
+            commonSourceSet 'customSourceSet'
+        }
+    }
+
+See more about multiplatform projects [here](https://kotlinlang.org/docs/reference/multiplatform.html).
+
 ## Tasks
 
 The Kotlin/Native plugin creates the following tasks:
@@ -243,6 +278,8 @@ for each an artifact defined in a `konanArtifacts` block. Such a task may have d
     |`enableOptimizations`|`boolean`                   |Are the optimizations enabled                             |
     |`enableAssertions   `|`boolean`                   |Is the assertion support enabled                          |
     |`measureTime        `|`boolean`                   |Does the compiler print phase time                        |
+    |`enableMultiplatform`|`boolean`                   |Is multiplatform support enabled for this artifact        |
+    |`commonSourceSet`    |`String`                    |Name of a source set used as a common module              |
 
     ##### Properties available for a cinterop task (task building an interoperability library):
 
@@ -359,7 +396,7 @@ tables below.
     // Language and API version.
     konan.languageVersion = 'version'
     konan.apiVersion = 'version'
-
+    
     konanArtifacts {
         // Targets to build this artifact for (optional, override the konan.targets list)
         program('foo', targets: ['android_arm32', 'android_arm64']) {
@@ -532,3 +569,26 @@ tables below.
         }
     }
 
+## Multiplatform DSL
+
+    apply plugin: 'konan'
+    
+    // In this example common code is located in 'bar' source set of ':common' project.
+
+    konanArtifacts {
+        // All the artifact types except interop libraries may use common modules.
+        program('foo') {
+            // All artifact settings described above are available here.
+            
+            // Enable multiplatform support for this artifact.
+            enableMultiplatform true
+            
+            // Set a custom name for a source set used as a common module.
+            commonSourceSet 'bar'
+        }
+    }
+
+    dependencies {
+        // Use the ':foo' project as a common project for multiplatform build.
+        expectedBy project(':common')
+    }

tools/kotlin-native-gradle-plugin/build.gradle

@@ -121,7 +121,7 @@ processResources {
     from(file("$rootBuildDirectory/utilities/env_blacklist"))
 }
 
-// TODO: Get rid of manual pom generation
+// TODO: Get rid of manual pom generation.
 publishing {
     publications {
         gradlePlugin(MavenPublication) {