Added README for target platform definition files

refs #7263

Change-Id: Idc8b8a8d87cc1b38be8cd2b963ba577a2fa30b05

diff --git a/releng/org.simantics.sdk.build.targetdefinition/README.md b/releng/org.simantics.sdk.build.targetdefinition/README.md
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+# Target platform definitions\r
+\r
+## org.simantics.sdk.build.targetdefinition.target\r
+\r
+The platform definition that is used for building the Simantics SDK itself.\r
+It does not include any of the parts of the org.simantics.sdk feature which\r
+defines the contents of the Simantics SDK.\r
+\r
+Essentially it includes all required Eclipse components and other third-party\r
+external components required for building the Simantics SDK.\r
+\r
+See ../org.simantics.sdk.build.p2.site/ for how some of the external third-party\r
+libraries are bundled for use with the Simantics SDK.\r
+\r
+## simantics.target\r
+\r
+Includes all the same components as *org.simantics.sdk.build.targetdefinition.target*\r
+but also includes the Simantics SDK which is sourced from [http://www.simantics.org/download/<branch>/sdk/](http://www.simantics.org/download/<branch>/sdk/).\r
+\r
+Each platform repository branch can have its own separate download location online, e.g:\r
+* [http://www.simantics.org/download/master/sdk/](http://www.simantics.org/download/master/sdk/)\r
+* [http://www.simantics.org/download/release/1.28.0/sdk/](http://www.simantics.org/download/release/1.28.0/sdk/)\r
+* [http://www.simantics.org/download/feature/scl/sdk/](http://www.simantics.org/download/feature/scl/sdk/)\r
+* ...\r
+\r
+which the simantics.target definition then references to get the SDK bundles.\r