add documentation on what each kiln release source can do

this is based on whether the release source implements component.ReleaseUploader

TILE_AUTHOR_GUIDE.md

@@ -234,11 +234,15 @@ I would recommend one set of credentials per release source type.
 
 #### <a id="release-source-boshio"></a> BOSH.io
 
+Kiln can only download releases from BOSH.io and can not upload BOSH Releases to BOSH.io.
+
 This release source has minimal configuration.
 Just add it to your `release_sources` and you can get releases from [BOSH.io](https://bosh.io/releases/).
 
 #### <a id='release-source-github'></a> GitHub Release Artifacts
 
+Kiln can only download releases from GitHub Releases and can not upload BOSH Releases to BOSH.io.
+
 To download BOSH Release Tarballs from GitHub Releases, add the following
 ```
 release_sources:
@@ -262,10 +266,14 @@ kiln fetch --variable="github_token=${GITHUB_TOKEN}"
 
 #### <a id='release-source-artifactory'></a> Build Artifactory
 
+Kiln can fetch and upload releases to/from Build Artifactory.
+
 _TODO_
 
 #### <a id='release-source-s3'></a> AWS S3
 
+Kiln can fetch and upload releases to/from AWS S3.
+
 _TODO_
 
 #### <a id='release-source-directory'></a> Local files