improved crosscompile

add README.md

README.md

@@ -0,0 +1,30 @@
+# platforms file
+
+## Crosscompiling
+```
+{
+  "Platforms": {
+    "Some crosscompile image": {
+      "enabled": true,
+      "codename": "<codename>",
+      "docker_dir": "<directory with Dockerfile>",
+      "docker_image": "<result docker image name>",
+      "docker_build_options": {
+        "image_from": "<source docker image name>",
+        "image_prepare_script": "<exec this script while building image>",
+        "arch": "<arch name>",
+        "toolchain": "<toolchain package name>",
+        "triplet": "<multiarch name>"
+      },
+      "cmake_toolchain": "<path to CMake toolchain file>",
+      "ppa_filters": ["<package name part to use in wildcard copy>"]
+    }
+  }
+}
+```
+
+[Debian codenames](https://wiki.debian.org/DebianReleases)
+[Debian arch names](https://wiki.debian.org/SupportedArchitectures)
+[Debian multiarch names](https://wiki.debian.org/Multiarch/Tuples)
+
+You can add any values in `docker_build_options` object, all these values passed to docker on building