add documntation

doc/building.rst

+Building with unibuild
+======================
+**Unibuild** need a build spec. Build spec must be bash script. *name* *version* *release* *summary* *description* variables and *_setup* *_build* *_install* functions uses. For example:
+
+.. code-block:: shell
+
+	#!/bin/bash
+	name="bash"
+	version="5.0"
+	summary="bash shell"
+	description="The bash shell"
+	source=(https://ftp.gnu.org/gnu/bash/bash-5.0.tar.gz)
+	_setup(){
+		export WORKDIR=$WORKDIR/example
+		./configure --prefix=/usr
+
+	}
+
+	_build(){
+		make
+	}
+
+	_install(){
+		make install DESTDIR=$INSTALLDIR
+	}
+	
+
+BUILDDIR
+^^^^^^^^
+Main directory that uses for all operations. Unibuild create 3 directory for building. **WORKDIR** **INSTALLDIR** **PKGDIR**
+All directories in builddir. Unibuild created builddir with **mktemp -d** command.
+
+1. WORKDIR
+
+Workdir is building directory. Before running functions, unibuild go workdir. Workdir is changeable variable. If only one source directory in workdir. unibuild automaticaly change workdir as source directory.
+
+2. INSTALLDIR
+
+Installdir is package destdir. Spec must store package files in here.
+
+3. PKGDIR
+
+Pkgdir is packaging directory required by target packager.
+
+Build functions
+^^^^^^^^^^^^^^^
+If you dont define functions, unibuild try to detect build method and create automaticaly. Or you can define **BuildType** variable for creating build function as automaticaly. Also you can define **CONFIG_OPTIONS** variable for auto build options. For example:
+
+.. code-block:: shell
+
+	#!/bin/bash
+	name="bash"
+	version="5.0"
+	summary="Bash shell"
+	description="The bash shell"
+	builddepends=("gcc")
+	depends=("readline")
+	license="gplv3"
+	source=(https://ftp.gnu.org/gnu/bash/bash-5.0.tar.gz)
+	BuildType="autotools"
+	
+This example uses autotools for compiling. autotools define build functions look like this:
+
+.. code-block:: shell
+
+	_setup(){
+		./configure --prefix=/usr $CONFIG_OPTIONS
+	}
+
+	_build(){
+		make -j$(nproc)
+	}
+
+	_install(){
+		make install DESTDIR=$INSTALLDIR
+	}
+	
+The gnu bash source code has ./configure script so if you dont define build functions, unibuild detect and set **BuildType** variable.
+
+Build target and host
+^^^^^^^^^^^^^^^^^^^^^
+Unibuild is universali builder so supported most of distribution. You can define **TARGET** and **HOST** variables.
+
+========    =================================================
+VARIABLE    DESCRIPION
+========    =================================================
+TARGET      Package build target: debian, inary, appimage ...
+HOST        Builder distribution: debian, inary ...
+========    =================================================
+
+host and target automaticaly detected if you dont define.
+
+Spec variables
+^^^^^^^^^^^^^^
+Unibuild spec variables and description avaiable here:
+
+========     ============    ========================================================
+OPTIONAL     VARIABLE        DESCRIPION
+========     ============    ========================================================
+no           name            Package name.
+no           version         Package version. Only can use [0-9] or . or -
+no           release         Package release. Muste be an integer.
+no           sources         Package source code url or path. Must be list.
+yes          executable      Package main executable name. Used by appimage
+no           description     Package description.
+no           summary         Package summary.
+yes          builddepends    Package names that required by compiling. must be array.
+yes          depends         Package runtime dependencies. must be array.
+no           license         Source code license.
+yes          partof          Package section or component name.
+========     ============    ========================================================
+
+Unibuild supported different source types. All known source types:
+
+1. name::git://xxxx.git
+
+2. git+https://xxxxx.git
+
+3. name::https://xxxxx
+
+4. /path/to/name/xxx
+
+5. https://xxxxxx
+