Added note about katex install for building docs

Currently, the version of katex that npm installs
and the version of nodejs that is installed by conda
from conda-forge are incompatible. This note describes
which version of katex to install to avoid this

[ghstack-poisoned]

CONTRIBUTING.md

@@ -467,12 +467,20 @@ pip install -r requirements.txt
 # Or if you prefer an uncontaminated global executable environment or do not want to go through the node configuration:
 # npm install katex && export PATH="$PATH:$(pwd)/node_modules/.bin"
 ```
+> Note: if you installed `nodejs` with a different package manager (e.g.,
+`conda`) then `npm` will probably install a version of `katex` that is not
+compatible with your version of `nodejs` and doc builds will fail.
+A combination of versions that is known to work is `node@6.13.1` and
+`katex@0.13.18`. To install the latter with `npm` you can run
+```npm install -g katex@0.13.18```
+
 
 > Note that if you are a Facebook employee using a devserver, yarn may be more convenient to install katex:
 
 ```bash
 yarn global add katex
 ```
+> If a specific version is required you can use for example `yarn global add katex@0.13.18`.
 
 3. Generate the documentation HTML files. The generated files will be in `docs/build/html`.
 

README.md

@@ -380,6 +380,13 @@ You can then build the documentation by running `make <format>` from the
 If you get a katex error run `npm install katex`.  If it persists, try
 `npm install -g katex`
 
+> Note: if you installed `nodejs` with a different package manager (e.g.,
+`conda`) then `npm` will probably install a version of `katex` that is not
+compatible with your version of `nodejs` and doc builds will fail.
+A combination of versions that is known to work is `node@6.13.1` and
+`katex@0.13.18`. To install the latter with `npm` you can run
+```npm install -g katex@0.13.18```
+
 ### Previous Versions
 
 Installation instructions and binaries for previous PyTorch versions may be found