Document how to publish to a maven repository (#1044)

* Create publish_to_maven.md

* add reference to maven publishing doc

* corrections after MIT license PR was merged

README.md

@@ -196,6 +196,9 @@ replacements:
         "@io_bazel_rules_scala_scala_xml//:io_bazel_rules_scala_scala_xml"
 ```
 
+## Publishing to Maven repository
+Go [here](docs/publish_to_maven.md)
+
 ## [Experimental] Dependency options
 
 There are a number of dependency options which can be set in the scala toolchain. These include `dependency_mode`, `strict_deps_mode`, `unused_dependency_checker_mode`, and `dependency_tracking_method`.

docs/publish_to_maven.md

@@ -0,0 +1,82 @@
+## Publish your Scala Libraries to a Maven Repository
+
+### 1. add a dependency on bazel-distribution repo from graknlabs
+```
+git_repository(
+    name = "graknlabs_bazel_distribution",
+    remote = "https://github.com/graknlabs/bazel-distribution",
+    commit = "e181add439dc1cfb7b1c27db771ec741d5dd43e6"
+)
+```
+
+### 2. add aggregate targets
+for each artifact you want to publish to maven central, 
+create an aggregate `scala_library` target that includes sources for all finer-grained targets (if you have
+targets with coarse-grained granularity you can skip this step)
+
+```python
+scala_library(
+    name = "greyhound-core",
+    srcs = [
+        "//core/src/main/scala/com/wixpress/dst/greyhound/core:sources",
+        "//core/src/main/scala/com/wixpress/dst/greyhound/core/admin:sources",
+        "//core/src/main/scala/com/wixpress/dst/greyhound/core/consumer:sources",
+        "//core/src/main/scala/com/wixpress/dst/greyhound/core/metrics:sources",
+        "//core/src/main/scala/com/wixpress/dst/greyhound/core/producer:sources",
+    ],
+    tags = ["maven_coordinates=com.wix:greyhound-core_2.12:{pom_version}"],
+    visibility = ["//:__subpackages__"],
+    deps = [
+        "@dev_zio_zio_2_12",
+        "@org_apache_kafka_kafka_clients",
+        "@org_slf4j_slf4j_api",
+    ],
+)
+```
+**A few notes:**
+1. All the labels in srcs are actually filegroup with appropriate visibility settings that allow out-of-package dependency:
+```python
+filegroup(
+   name = "sources",
+   srcs = glob(["*.java"]) + glob(["*.scala"]),
+   visibility = ["//core:__subpackages__"],
+)
+```
+2. There is a special `maven_coordinates` tag that helps the `assemble_maven` rule to fill-in details in the pom file.
+3. You also have to add a `maven_coordinates` tag to the 3rd party dependencies (such as `dev_zio_zio_2_12` in the deps attribute of the example above) so that they will appear as dependencies in the pom file.
+
+### 3. Add `assemble_maven` target
+Add assemble_maven target for each artifact you want to publish. 
+It will create all the necessary files for your artifact, including main jar, source jar and pom file. Enter all project details for the pom file.
+```python
+load("@graknlabs_bazel_distribution//maven/templates:rules.bzl", "deploy_maven", "assemble_maven")
+
+assemble_maven(
+    name = "assemble-maven",
+    target = "//core:greyhound-core",
+    package = "{maven_packages}",
+    project_name = "Greyhound Core",
+    project_description = "Greyhound - Rich Kafka Client with ZIO API",
+    project_url = "https://github.com/wix/greyhound",
+    scm_url = "https://github.com/wix/greyhound.git",
+    version_file = "//central-sync:VERSION",
+    developers = {"1": ["name=Natan Silnitsky", "email=n...@w...m", "organization=Wix"]},
+    license = "mit"
+)
+```
+Notes:
+1. For the target attribute you should put the label for the `scala_library` target you created in the previous step with all the relevant sources.
+2. Make sure the `project_name` and `project_description` are unique for each of these targets/artifacts
+3. The `VERSION` file just contains the SEMVER, e.g. 1.0.0
+4. Currently supported licenses include `apache` and `MIT`
+
+### 4. Add deploy_maven target
+Add `deploy_maven` target for each artifact you want to publish.
+The `deployment.properties` file should contain the url for the sonatype stating area:
+repo.maven.release=https://oss.sonatype.org/service/local/staging/deploy/maven2/
+
+### 5. Build the assemble_maven target 
+Build the assemble_maven target and review the generated pom file and jar file in the bazel-bin directory. Change the targets configuration as needed.
+<br/>
+
+For more specific information on publishing to Maven Central repository, see [this blog post](https://medium.com/wix-engineering/how-to-publish-artifacts-from-bazel-to-maven-central-71da0cf990f5)