[doc] Rework README.md on how to get CAISAR.

Now mentions the opam package just released.

CHANGES.md

+## 0.1.1 (Unreleased)
+
+- Rework the section in `README.md` for obtaining CAISAR in order to mention the
+  `opam` package just released.
+
 ## 0.1 (13-07-2022)
 
 First public release of CAISAR.

README.md

@@ -5,30 +5,21 @@ under active development at CEA LIST, aiming to provide a
 wide range of features to characterize the safety and robustness of
 artificial intelligence based software.
 
-## Installation
+## Getting CAISAR
 
-No binaries are provided at the moment. Installation must be done by either
-compiling the source code or using a [Docker](https://www.docker.com/) image.
+The latest release of CAISAR is available as an [opam](https://opam.ocaml.org/)
+package or a [Docker](https://www.docker.com/) image.
 
-### From source code
+The development version of CAISAR is available only by compiling the source code.
+
+### opam package
 
 **Please note:** CAISAR requires the OCaml package manager [opam](https://opam.ocaml.org/),
 v2.1 or higher, which is typically avaible in all major GNU/Linux distributions.
 
-To build and install CAISAR, do the following:
-
-```
-$ git clone https://git.frama-c.com/pub/caisar
-$ cd caisar
-$ opam switch create --yes --no-install . 4.13.1
-$ opam install . --deps-only --with-test --yes
-$ make
-$ make install
-```
-
-To run the tests:
+To install CAISAR via [opam](https://opam.ocaml.org/), do the following:
 ```
-$ make test
+$ opam install caisar
 ```
 
 ### Docker image
@@ -53,6 +44,27 @@ To run the CAISAR [Docker](https://www.docker.com/) image, do the following:
 $ docker run -it laiser/caisar:pub sh
 ```
 
+### From source code
+
+**Please note:** CAISAR requires the OCaml package manager [opam](https://opam.ocaml.org/),
+v2.1 or higher, which is typically avaible in all major GNU/Linux distributions.
+
+To build and install CAISAR, do the following:
+
+```
+$ git clone https://git.frama-c.com/pub/caisar
+$ cd caisar
+$ opam switch create --yes --no-install . 4.13.1
+$ opam install . --deps-only --with-test --yes
+$ make
+$ make install
+```
+
+To run the tests:
+```
+$ make test
+```
+
 ## Usage
 
 To start using CAISAR, please run the command: