README improvements

Various clarifications and reformulations based on my experience using
the README to build and run Gargantext. In particular:
- Information related to development has been given its own section
- Docker Compose has been added to the list of prerequisites
- Information pertaining to using Stack has been moved to an annex file
- The content to be copypasted into a `cabal.project.local` file has been moved
  out of the README into a separate file `cabal.project.local_toCopy`

Those may not be the best decisions, I am happy to discuss them if
needed

cabal.project.local_toCopy

+package gargantext
+    ghc-options: -fwrite-ide-info -hiedir=".stack-work/hiedb" -O0
+
+package gargantext-admin
+    ghc-options: -O0
+
+package gargantext-cli
+    ghc-options: -O0
+
+package gargantext-db-obfuscation
+    ghc-options: -O0
+
+package gargantext-import
+    ghc-options: -O0
+
+package gargantext-init
+    ghc-options: -O0
+
+package gargantext-invitations
+    ghc-options: -O0
+
+package gargantext-phylo
+    ghc-options: -O0
+
+package gargantext-server
+    ghc-options: -O0
+
+package gargantext-upgrade
+    ghc-options: -O0
+
+package gargantext-graph
+    ghc-options: -O0
+
+package hmatrix
+    ghc-options: -O0
+
+package sparse-linear
+    ghc-options: -O0
+

docs/using_stack.md

+# Building Gargantext using the Stack tool
+
+Those are the instructions for developers who wish to build Gargantext using stack instead of cabal.
+
+## Prerequisites
+
+You need [Stack](https://docs.haskellstack.org/en/stable/) (obviously). You can install it with:
+```shell
+curl -sSL https://get.haskellstack.org/ | sh
+```
+
+Check that the installation is complete with:
+```shell
+stack --version
+Version 2.9.1
+```
+
+## Building and setting up
+
+To build with stack, follow the instructions in `README.md`, with the following changes:
+
+- Replace the `cabal update` and `cabal install` commands with (still from within a Nix shell!):
+```shell
+stack build --fast
+```
+
+*Note: The default build (with optimizations) requires large amounts of RAM (16GB at least). The (recommended) `--fast` flag is here to avoid heavy compilation times and swapping out your machine; just omit it if you want to build with optimizations.*
+
+- After you have run the `docker compose up` command, install with
+```shell
+stack install
+```
+
+## Keeping the stack.yaml updated with the cabal.project
+
+Once you have a valid version of stack, building requires generating a valid `stack.yaml`. This can be obtained by installing `cabal2stack`:
+```shell
+git clone https://github.com/iconnect/cabal2stack.git
+cd cabal2stack
+```
+
+Then, depending on what build system you are using, either build with `cabal install --overwrite-policy=always` or `stack install`.
+And finally:
+```shell
+cabal2stack --system-ghc --allow-newer --resolver lts-21.17 --resolver-file devops/stack/lts-21.17.yaml -o stack.yaml
+stack build
+```
+
+The good news is you don't have to do all of this manually; during development, after modifying `cabal.project`, it's enough to run:
+```shell
+./bin/update-project-dependencies
+```