Start a "conventions" section to span all the specs

It's short for now, but we will extract more and more things from
the rest of the specifications as we continue editing.

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+# Notation and conventions
+
+These conventions apply,
+at least in theory,
+to all of the specification documents
+unless stated otherwise.
+
+> Remember, our specification documents
+> were once a collection of separate text files,
+> written separately
+> and edited over the course of years.
+>
+> While we are trying (as of 2023)
+> to edit them into consistency,
+> you should be aware that these conventions
+> are not now followed uniformly everywhere.
+
+## MUST, SHOULD, and so on {#rfc2119}
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
+NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
+"OPTIONAL" in this document are to be interpreted as described in
+[RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119).
+
+## Data lengths {#data-lengths}
+
+Unless otherwise stated,
+all lengths are given as a number of 8-bit bytes.
+
+> All bytes are 8 bits long.
+> We sometimes call them "octets";
+> the terms as used here are interchangeable.
+
+When referring to longer lengths,
+we use [SI binary prefixes](https://en.wikipedia.org/wiki/Binary_prefix)
+(as in "kibibytes", "mebibytes", and so on)
+to refer unambiguously to increments of 1024<sup>X</sup> bytes.
+
+> If you encounter a reference
+> to "kilobytes", "megabytes", or so on,
+> you cannot safely infer whether the author intended
+> a decimal (1000<sup>N</sup>) or binary (1024<sup>N</sup>) interpretation.
+> In these cases, it is better to revise the specifications.
+
+<a id="tor-spec.txt-0.1.1"></a>
+
+## Integer encoding {#integers}
+
+Unless otherwise stated,
+all multi-byte integers are encoded
+in big-endian ("network") order.
+
+> For example, 4660 (0x1234),
+> when encoded as a two-byte integer,
+> is the byte 0x12 followed by the byte 0x34. (\[12 34\])
+>
+> When encoded as a four-byte integer,
+> it is the byte 0x00, the byte 0x00, the byte 0x12, and the byte 0x34.
+> (\[00 00 12 34\]).
+
+## Notation {#notation}
+
+### Binary literals {#binary-literals}
+
+When we write a series of one-byte hexadecimal literals
+in square brackets,
+it represents a multi-byte binary string.
+
+> For example,
+> `[6f 6e 69 6f 6e 20 72 6f 75 74 69 6e 67]`
+> is a 13-byte sequence representing the unterminated ASCII string,
+> `onion routing`.
+
+
+
+
+