Updated documentation about building development version.

src/site/markdown/building.md

@@ -35,8 +35,9 @@ For systems not providing maven as a package, maven can be
 Apache Software Foundation. This site also explains the
 installation procedure.
 
-As with all maven enabled projects, building Orekit is straightforward, simply
-run:
+As with all maven enabled projects, building official released versions of
+Orekit is straightforward (see below for the special case of development versions),
+simply run:
 
     mvn assembly:single
 
@@ -44,12 +45,38 @@ The preceding command will perform all dependencies retrieval, compilation,
 tests and packaging for you. At the end, it will create several files named
 target/orekit-x.y.jar where x.y is the version number.
 
+This command should always work for released Orekit versions as they
+always depend only on released Apache Commons Math versions. Maven knows how
+to download the pre-built binary for released Apache Commons Math versions.
+The previous command may not work for development Orekit versions as they
+may depend on unreleased Apache Commons Math versions. Maven cannot download
+pre-built binaries for unreleased Apache Commons Math versions as none are
+publicly available. In this case the command above will end with an error message
+like:
+
+    [ERROR] Failed to execute goal on project orekit: Could not resolve dependencies for project org.orekit:orekit:jar:7.1-SNAPSHOT: Could not find artifact org.apache.commons:commons-math3:jar:3.6-SNAPSHOT
+
+In this case, you should build the missing Apache Commons Math artifact and
+install it in your local maven repository beforehand. This is done by cloning
+the Apache Commons Math source (using the MATH_3_X branch) from Apache git
+repository in some temporary folder and install it with maven. This is done by
+running the commands below (using Linux command syntax):
+
+    git clone --branch MATH_3_X https://git-wip-us.apache.org/repos/asf/commons-math.git
+    cd commons-math
+    mvn install
+
+Once the Apache Commons Math development version has been installed locally using
+the previous commands, you can delete the cloned folder if you want. You can then
+attempt again the mvn command at Orekit level, this time it should succeed as the
+necessary artifact is now locally available.
+
 If you need to configure a proxy server for dependencies retrieval, see
 the [Guide to using proxies](http://maven.apache.org/guides/mini/guide-proxies.html)
 page at the maven site.
 
 If you already use maven for your own projects (or simply eclipse, see
-below), you may want to install Orekit in your local repository. This is done
+below), you may want to install Orekit in your local maven repository. This is done
 with the following command:
 
     mvn install

src/site/markdown/faq.md

@@ -111,6 +111,33 @@ with version 4.1, Orekit depends only on officially released versions of Apache
   Orekit 6.1   | Apache Commons Math 3.2
   Orekit 7.0   | Apache Commons Math 3.4.1
 
+### Maven failed to compile Orekit and complained about a missing artifact.
+
+The released versions of Orekit always depend only on released Apache Commons
+Math versions, but development Orekit versions may depend on unreleased Apache
+Commons Math versions. Maven knows how to download the pre-built binary for
+released Apache Commons Math versions but it cannot download
+pre-built binaries for unreleased Apache Commons Math versions as none are
+publicly available. In this case the mavan command will end with an error message
+like:
+
+    [ERROR] Failed to execute goal on project orekit: Could not resolve dependencies for project org.orekit:orekit:jar:7.1-SNAPSHOT: Could not find artifact org.apache.commons:commons-math3:jar:3.6-SNAPSHOT
+
+In this case, you should build the missing Apache Commons Math artifact and
+install it in your local maven repository beforehand. This is done by cloning
+the Apache Commons Math source (using the MATH_3_X branch) from Apache git
+repository in some temporary folder and install it with maven. This is done by
+running the commands below (using Linux command syntax):
+
+    git clone --branch MATH_3_X https://git-wip-us.apache.org/repos/asf/commons-math.git
+    cd commons-math
+    mvn install
+
+Once the Apache Commons Math development version has been installed locally using
+the previous commands, you can delete the cloned folder if you want. You can then
+attempt again the mvn command at Orekit level, this time it should succeed as the
+necessary artifact is now locally available.
+
 ### The orekit-data.zip file you provide is not up to date. Can you update it?
 
 There is no regular update for this file. Data are provided only as an example, to allow quick