Add docs: how to use the Pipfile and to update this too

Pipfile

@@ -1,4 +1,4 @@
-# Note that there is a devrequirements.txt file in this project, if you are 
+# Note that there is a dev-requirements.txt file in this project, if you are 
 # updating this Pipfile make sure to update the devrequirements.txt accordingly
 
 [dev-packages]

docs/source/contributing/contributing.md

@@ -67,8 +67,11 @@ make that your current directory with `cd repo2docker`.
 
 After cloning the repository (or your fork of the repository), you should set up an
 isolated environment to install libraries required for running / developing
-repo2docker. There are many ways to do this, and a `virtual environment` is
-one of them.
+repo2docker. 
+
+There are many ways to do this but here we present you with two approaches: `virtual environment` or `pipenv`.
+
+- Using `virtual environment`
 
 ```bash
 python3 -m venv .
@@ -80,6 +83,19 @@ pip3 install -r docs/doc-requirements.txt
 
 This should install all the libraries required for testing & running repo2docker!
 
+- Using `pipenv`
+
+Note that you will need to install pipenv first using `pip3 install pipenv`. 
+Then from the root directory of this project you can use the following commands:
+
+```bash
+pipenv install --dev
+```
+
+This should install both the dev and docs requirements at once!
+
+### Set up 
+
 ### Verify that docker is installed and running
 
 If you do not already have [Docker](https://www.docker.com/), you should be able

docs/source/contributing/tasks.md

@@ -149,3 +149,15 @@ Some details:
 Release candidate versions in the change log are only temporary, and
 should be superseded by either a next release candidate, or the final
 release for that version (bugfix version 0).
+
+
+### Keeping the Pipfile and requirements files up to date
+We now have both a `dev-requirements.txt` and a `Pifile` for repo2docker, as 
+such it is important to keep these in sync/up-to-date. 
+
+Both files use `pip identifiers` so if you are updating for example the Sphinx version 
+in the `doc-requirements.txt` (currently `Sphinx = ">=1.4,!=1.5.4"`) you can use the
+same syntax to update the Pipfile and viceversa.
+
+At the moment this has to be done manually so please make sure to update both
+files accordingly.