Readme: move python packaged to the detailed one. (acl-org#523)

Make handles installation of packages now so the average contributor
does not need to be burdened with the installation of python packages.

Make libyaml installation more explicit.

README.md

@@ -9,18 +9,16 @@ The offical home of this repository is <https://github.com/acl-org/acl-anthology
 
 To build the Anthology website, you will need:
 
-+ **Python 3.7** or higher, along with the packages listed in
-  [bin/requirements.txt](bin/requirements.txt)
-  + *Note:* You can install all needed dependencies using the command `pip install -r bin/requirements.txt`
-  + *Note:* [Installing the PyYAML package with C
-    bindings](http://rmcgibbo.github.io/blog/2013/05/23/faster-yaml-parsing-with-libyaml/)
-    will speed up the generation process.
++ **Python 3.7** or higher
 + [**Hugo 0.54**](https://gohugo.io) or higher (can be [downloaded directly from
   their repo](https://github.com/gohugoio/hugo/releases); the ***extended version*** is required!)
 + [**bibutils**](https://sourceforge.net/p/bibutils/home/Bibutils/) for creating
   non-BibTeX citation formats (not strictly required to build the website, but
   without them you need to invoke the build steps manually as laid out in the
   [detailed README](README_detailed.md))
++ *optional*: If you install `libyaml-dev` before running `make` the first time,
+   the libyaml C library will be used instead of a python implementation, speeding up
+   the build.
 
 ### Cloning
 

README_detailed.md

@@ -11,6 +11,17 @@ site generator.  However, before we can actually invoke Hugo, we need to prepare
 the contents of the website.  The following steps describe what happens
 behind the scenes.  All the steps have a corresponding `make` target as well.
 
+### Step 0: Install required Python packages
+To build the anthology, the packages listed in
+  [bin/requirements.txt](bin/requirements.txt) are needed (they are installed and updated by make automatically).
+  + *Note:* You can install all needed dependencies using the command `pip install -r bin/requirements.txt`
+  + *Note:* [Installing the PyYAML package with C
+    bindings](http://rmcgibbo.github.io/blog/2013/05/23/faster-yaml-parsing-with-libyaml/)
+    will speed up the generation process.  On Debian-based systems, you have to do
+	the following if `libyaml-dev` was not installed before running make the first time:
+	`sudo apt install libyaml-dev`, enable virtualenv: `source venv/bin/activate` and
+	rebuild pyyaml with libyaml backend: `pip3 install pyyaml --upgrade --force`
+
 ### Step 1: Prepare the data for site generation
 
 The data sources for the Anthology currently reside in the [`data/`](data/)