Added a read for the SDK project

readme.md

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+# jMonkeyEngine SDK
+
+This repo holds the Legacy SDK of jMonkeyEngine, based on Netbeans platform.
+
+## Getting started
+You'll need several things to have the SDK build and run :
+- java, if you don't know what it is or how to install it... you'd better run from here.
+- [gradle](http://gradle.org/gradle-download/) to build the project.
+- [NetBeans](https://netbeans.org/downloads/) to edit and run the project (javaSE distrib should be enough).
+- Then you need to read this [documentation](http://wiki.jmonkeyengine.org/doku.php/sdk:development) thoroughly.
+
+Before hopping into Netbeans plugin development you'll need to build the project using gradle command :
+```
+gradlew buildSdk
+```
+
+Then you'll be able to open the project in netbeans and start your devs.
+
+## Creating a distribution of the SDK
+When in Netbeans right click on the SDK project and choose : **package as...**
+then choose whatever distribution you fancy.
+![Package as...](http://i.imgur.com/5V2uBHf.png)
+
+## jMonkeyEngine3 version
+The sdk uses jME published artifacts.
+You can change the version of these artifacts by editing the build.gradle file and changinf the ext.jmeFullVersion variable to a proper version.
+
+### Using jME official release
+jME official release are published on [jcenter](https://bintray.com/bintray/jcenter). You'll find there all the releases listed [here](https://github.com/jMonkeyEngine/jmonkeyengine/releases) since jME 3.1.0-alpha2
+To use jcenter as a repository just put :
+```
+jcenter()
+```
+or
+```
+jcenter {
+    url "http://jcenter.bintray.com/"
+}
+```
+in the repositories section of the build.gradle.
+Then you have to change the jmeFullVersion to whatever official release version.
+
+<i>example : 3.1.0-alpha2</i>
+
+### Using jME SNAPSHOT versions
+jME is built on each commit, and a SNAPSHOT version is done and published on a [custom public repo](http://updates.jmonkeyengine.org/maven/)
+To use this repository just put :
+```
+maven {
+    url "http://updates.jmonkeyengine.org/maven/"
+}
+```
+in the repositories section of the build.gradle.
+Then you have to change the jmeFullVersion to a SNAPSHOT version.
+
+<i>example : 3.1.0-SNAPSHOT</i>
+> WARNING !!! Note that depending on how often gradle updates your dependencies, using SNAPSHOT version can break your build any time
+
+### Using jME from any git branch or commit
+You can use [jitpack](https://jitpack.io/) as a repository to be able to build jME dependencies from any branch or commit.
+To use jitpack as a repository just put :
+
+```
+maven {
+    url "https://jitpack.io"
+}
+```
+in the repositories section of the build.gradle.
+Then you have to change the jmeFullVersion to a branch or commit tag (see [jitpack documentation](https://jitpack.io/docs/)).
+
+<i>example : PBRisComing-SNAPSHOT</i>
+> WARNING !!! Note that depending on how often gradle updates your dependencies, using branch or commit dependency version can break your build any time