Add documentation section to the README.

My goal with this is to funnel people towards immediate, quick answers
first; PHP-specific guidance second; and the C documentation (which
contains a lot of technical information but isn't super useful for
PHP developers) next.

If we can keep people off Google/StackOverflow, we're doing a good job.

README.md

@@ -232,6 +232,18 @@ and the slowdown will be greatly reduced.
 This is also true of non-Windows 32-bit operating systems, or if somehow PHP
 was compiled where `PHP_INT_SIZE` equals `4` instead of `8` (i.e. Linux on i386).
 
+## Documentation
+
+First, you'll want to read the [Libsodium Quick Reference](https://paragonie.com/blog/2017/06/libsodium-quick-reference-quick-comparison-similar-functions-and-which-one-use).
+It aims to answer, "Which function should I use for [common problem]?".
+
+If you don't find the answers in the Quick Reference page, check out
+[*Using Libsodium in PHP Projects*](https://paragonie.com/book/pecl-libsodium).
+
+Finally, the [official libsodium documentation](https://download.libsodium.org/doc/) 
+(which was written for the C library, not the PHP library) also contains a lot of
+insightful technical information you may find helpful.
+
 ## API Coverage
 
 **Recommended reading:** [Libsodium Quick Reference](https://paragonie.com/blog/2017/06/libsodium-quick-reference-quick-comparison-similar-functions-and-which-one-use)
@@ -323,7 +335,6 @@ was compiled where `PHP_INT_SIZE` equals `4` instead of `8` (i.e. Linux on i386)
   `ParagonIE_Sodium_Compat::crypto_pwhash_is_available()`, which returns a
    boolean value (`TRUE` or `FALSE`).
 
-
 ### PHPCompatibility Ruleset
 
 For sodium_compat users and that utilize [`PHPCompatibility`](https://github.com/PHPCompatibility/PHPCompatibility)