[readme] add headings to certain sections

Fixes #2413.

README.md

@@ -27,6 +27,9 @@
   - [Setting Custom Colors](#setting-custom-colors)
     - [Persisting custom colors](#persisting-custom-colors)
     - [Suppressing colorized output](#suppressing-colorized-output)
+    - [Restoring PATH](#restoring-path)
+    - [Set default node version](#set-default-node-version)
+    - [Use a mirror of node binaries](#use-a-mirror-of-node-binaries)
   - [.nvmrc](#nvmrc)
   - [Deeper Shell Integration](#deeper-shell-integration)
     - [bash](#bash)
@@ -440,18 +443,21 @@ nvm help --no-colors
 TERM=dumb nvm ls
 ```
 
+#### Restoring PATH
 To restore your PATH, you can deactivate it:
 
 ```sh
 nvm deactivate
 ```
 
+#### Set default node version
 To set a default Node version to be used in any new shell, use the alias 'default':
 
 ```sh
 nvm alias default node
 ```
 
+#### Use a mirror of node binaries
 To use a mirror of the node binaries, set `$NVM_NODEJS_ORG_MIRROR`:
 
 ```sh