Update conda docs (#164)

* Add instructions for conda update --all

* Better wording

* Bit more clarity

* Add a bit more detail as suggested by @clyne

Author: brian-rose

CONTRIBUTING.md

@@ -27,7 +27,14 @@ Use [conda](https://docs.conda.io/) to set up a build environment:
 conda env update -f ../ci/environment.yml
 ```
 
-This will create or update the dev environment (`pythia`).
+This will create the dev environment (`pythia`). If you have previously created the environment, running this command will add any new packages that have since been added to the `environment.yml` file.
+
+It's a good idea to also keep the *versions* of each package in the `pythia` environment up to date by doing:
+
+```bash
+conda activate pythia
+conda update --all
+```
 
 #### Install `pre-commit` hooks
 
@@ -52,6 +59,8 @@ Build the site locally using Sphinx (which you just installed in the `pythia` en
 make html
 ```
 
+If this step fails and you have not updated your conda environment recently, try updating with `conda env update -f ../ci/environment.yml` and `conda update --all` as described above.
+
 The newly rendered site is now available in `portal/_build/html/index.html`.
 Open with your web browser, or from the terminal:
 
@@ -78,7 +87,12 @@ When you're done, you can deactivate the dedicated build environment with
 conda deactivate
 ```
 
-You can re-activate the `pythia` conda environment at any time with `conda activate pythia`.
+You can re-activate the `pythia` conda environment at any time with `conda activate pythia`. You may also want to update each package in the activated environment to their latest versions by doing
+
+```bash
+conda activate pythia
+conda update --all
+```
 
 ### To view the Read the Docs autobuild