docs: Explain exchange rate terminology. (#270)

In particular, what does "X/Y" mean? What are "base" and "quote" assets?

In short in "X/Y", X is the "base" asset, and Y is the "quote" asset,
and it tells you the price of X in terms of Y.

At least for me, this is really not self-explanatory.

src/ic-xrc-types/src/lib.rs

@@ -19,13 +19,16 @@ pub struct Asset {
 }
 
 /// The type the user sends when requesting a rate.
+///
+/// For definitions of "base", "quote", etc, the reader is referred to
+/// https://en.wikipedia.org/wiki/Currency_pair.
 #[derive(CandidType, Clone, Debug, Deserialize)]
 pub struct GetExchangeRateRequest {
-    /// The asset to be used as the resulting asset. For example, using
-    /// ICP/USD, ICP would be the base asset.
+    /// The base asset, i.e., the first asset in a currency pair. For example,
+    /// ICP is the base asset in the currency pair ICP/USD.
     pub base_asset: Asset,
-    /// The asset to be used as the starting asset. For example, using
-    /// ICP/USD, USD would be the quote asset.
+    /// The quote asset, i.e., the second asset in a currency pair. For example,
+    /// USD is the quote asset in the currency pair ICP/USD.
     pub quote_asset: Asset,
     /// An optional parameter used to find a rate at a specific time.
     pub timestamp: Option<u64>,