Fix help and write first version of README

README.md

@@ -1,2 +1,57 @@
 # emacs-build
+
 Scripts to build a distribution of Emacs from sources, using MSYS2 and Mingw64(32)
+
+# Rationale
+
+I wanted a script to build Emacs from sources, package it and install it on
+different computers, with the following conditions
+
+- I should be able to build any branch or release from Emacs. This includes
+  the last release branch (right now emacs-27), as well as the master branch
+  for development. Always using pristine sources from Savannah.
+
+- I want to build emacs with different options from the default, which is to
+  use all features available. For instance, I do not care for SVG support.
+
+- The script needs to track all packages that are required by the Emacs build
+  even if I change the build options.
+
+- The installation should take as little space as possible, removing useless
+  directories or files that come from the dependencies. For instance, headers
+  from libraries used by emacs, spurious documentation files, etc.
+
+- Eventually, the script should be able to build other components I regularly
+  use, such as mu, mu4e or pdf-tools.
+
+# Usage
+
+````
+   ./emacs-build.sh [-64] [-32] [--branch b]
+                    [--clone] [--ensure] [--build] [--deps] [--package]
+                    [--without-X] [--with-X]
+
+Actions:
+
+   --clone       Download Savannah's git repository for Emacs
+   --ensure      Ensure that required packages are installed
+   --build       Configure and build Emacs from sources
+   --deps        Create a ZIP file with all the Mingw64/32 dependencies
+   --package     Package an Emacs previously built with the --build option
+
+   Multiple actions can be selected. The default is to run them all in a logical
+   order: clone, ensure, build, deps and package.
+
+Options:
+   -64           Prepare or build for Mingw64 (default)
+   -32           Prepare or build for Mingw32
+   --branch b    Select branch 'b' for the remaining operations
+   --slim        Remove Cairo, SVG and TIFF support for a slimmer build
+                 Remove also documentation files and other support files
+                 from the dependencies file
+   --with-X      Add requested feature in the dependencies and build
+   --without-X   Remove requested feature in the dependencies and build
+
+   X is any of the known features for emacs in Windows/Mingw:
+     $all_features
+````

emacs-build.sh

@@ -32,7 +32,9 @@ function write_help () {
     cat <<EOF
 Usage:
 
-   ./emacs-build.sh [-64] [-32] [--clone] [--build] [--deps] [--without-X] [--with-X]
+   ./emacs-build.sh [-64] [-32] [--branch b]
+                    [--clone] [--ensure] [--build] [--deps] [--package]
+                    [--without-X] [--with-X]
 
 Actions:
 
@@ -48,6 +50,7 @@ Actions:
 Options:
    -64           Prepare or build for Mingw64 (default)
    -32           Prepare or build for Mingw32
+   --branch b    Select branch 'b' for the remaining operations
    --slim        Remove Cairo, SVG and TIFF support for a slimmer build
                  Remove also documentation files and other support files
                  from the dependencies file