Added algorithm.md, informal explanation of emph/link parsing.

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+Algorithm for parsing nested links, images, emphasis, and quotes
+================================================================
+
+When we're parsing inlines and we hit
+
+- a run of `*` or `_` characters
+- a `[` or `![`
+
+we insert a text node with the literal content, and add a pointer
+to this text node to the **delimiter stack.**
+
+The **delimiter stack** is a doubly linked list.  Each
+element contains a pointer to a text node, plus information about
+
+- the type of delimiter (`[`, `![`, `*`, `_`)
+- the number of delimiters,
+- whether the delimiter is "active" (all are active to start), and
+- whether the delimiter is a potential opener, a potential closer,
+  or both.
+
+When we hit a `]` character, we call the `look_for_link_or_image`
+procedure (see below).
+
+When we hit the end of the input, we call the `process_emphasis`
+procedure (see below), with `stack_bottom` = NULL.
+
+`look_for_link_or_image`
+------------------------
+
+Starting at the top of the delimiter stack, we look backwards
+through the stack for a `[` or `![` delimiter.
+
+If we don't find one, we return a literal text node `]`.
+
+If we do find one, but it's not *active*, we remove the inactive
+delimiter from the stack, and return a literal text node `]`.
+
+If we find one and it's active, then we parse ahead to see if
+we have an inline link/image, reference link/image, compact reference
+link/image, or shortcut reference link/image.
+
+If we don't, then we remove the `[` or `![` delimiter from the
+delimiter stack and return a literal text node `]`.
+
+If we do, then
+
+- We return a link or image node whose children are the inlines
+  after the text node pointed to by the opening delimiter.
+
+- We run `process_emphasis` on these inlines, with the `[` opener
+  as `stack_bottom`.
+
+- We remove the opening delimiter.
+
+- If we have a link (and not an image), we also set all
+  `[` delimiters before the opening delimiter to *inactive*.  (This
+  will prevent us from getting links within links.)
+
+
+`process_emphasis`
+------------------
+
+Parameter `stack_bottom` sets a lower bound to how far we
+descend in the **delimiter stack**.  If it is NULL, we can
+go all the way to the bottom.  Otherwise, we stop before
+visiting `stack_bottom`.
+
+Let `current_position` point to the element on the delimiter
+just above `stack_bottom` (or the first element if `stack_bottom`
+is NULL).
+
+We keep track of the `openers_bottom` for each delimiter
+type (`*`, `_`).  Initialize this to `stack_bottom`.
+
+Then we repeat the following until we run out of potential
+closers:
+
+- Move `current_position` forward in the delimiter stack (if needed)
+  until we find the first potential closer with delimiter `*` or `_`.
+  (This will be the potential closer closest
+  to the beginning of the input -- the first one in parse order.)
+
+- Now, look back in the stack (staying above `stack_bottom` and
+  the `openers_bottom` for this delimiter type) for the
+  first matching potential opener ("matching" means same delimiter).
+
+- If one is found:
+
+  - Figure out whether we have emphasis or strong emphasis:
+    if both closer and opener spans have length >= 2, we have
+    strong, otherwise regular.
+  - Insert an emph or strong emph node accordingly, after
+    the text node corresponding to the opener.
+  - Remove delimiters between opener and closer from the delimiter
+    stack.
+  - Remove 1 (for regular emph) or 2 (for strong emph) delimiters
+    from the opening and closing text nodes.  If they become empty
+    as a result, remove them and remove the corresponding element
+    of the delimiter stack.  If the closing node is removed, reset
+    `current_position` to the next element in the stack.
+
+- If none in found:
+
+  - Set `openers_bottom` to the element before `current_position`.
+    (We know that there are no openers for this kind of closer up to and
+    including this point, so this puts a lower bound on future searches.)
+  - If the closer at `current_position` is not a potential opener,
+    remove it from the delimiter stack (since we know it can't
+    be a closer either).
+  - Advance `current_position` to the next element in the stack.
+
+- Repeat.
+
+- After we're done, remove all delimiters above `stack_bottom` from the
+  delimiter stack.
+