Add syntax.md. The first version of the syntax.

syntax.md

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+# syntax
+
+How to define macros and what not to define.
+
+## definition of simple macros
+Macros are defined in a separate file, by default called `macros`.
+You can change/add additional macro files through the command line options.
+
+Examples of macro definitions:
+
+```
+.hc hydrocarbon
+.h hydrogen
+.c carbon
+```
+
+### external definitions
+
+You can also `source` macros from another macro file:
+
+```
+source ../macros
+source ../../macros
+.hc hydrocarbons
+.h hydrogen
+.c
+```
+
+This sources macros from its parent and grandparent directories.
+This may be useful if you have several related topics in separate folders,
+but only some of the macros are shared and some are not.
+
+## plurals
+
+Say you only have the following macro definitions:
+
+`.hc hydrocarbon`
+
+And the file `test.md`:
+
+```markdown
+# .Hc
+In organic chemistry, a .hc is an organic compound consisting entirely of 
+.h and .c. .Hcs are examples of group 14 hydrides. 
+```
+
+The output would be the following:
+
+```markdown
+# Hydrocarbon
+In organic chemistry, a hydrocarbon is an organic compound consisting entirely
+of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. 
+```
+
+Note how `.hcs` becomes `hydrocarbons` even though that isn't explicitly
+defined. This helps you avoid adding the plural forms of words.
+
+
+### other default plurals 
+
+There are also other default plurals:
+
+Ending | Example | Formation | Plural
+--- | --- | --- | ---
+`s`, `ch`, `sh`, `x`, or `z` | dress | add es | dresses
+`[consonant]y` | story | change `y` to `ies` | stories
+`[consonant]o` | hero | add `es` | heroes
+`f` | leaf | change `f` to `ves` | leaves
+
+
+### custom plurals
+You can also define your own custom plurals:
+
+`macros`
+```
+.s salmon salmon
+.m man men
+.l louse lice
+```
+
+Input:
+```
+Hello boys and girls, my name is Fat Lip
+and this is my friend Sammy the .S.
+(What 'do?)
+Today, we're going to teach you some fun new facts about .ss
+and a brand new dance.
+```
+
+Output:
+```
+Hello boys and girls, my name is Fat Lip
+and this is my friend Sammy the Salmon.
+(What 'do?)
+Today, we're going to teach you some fun new facts about salmon
+and a brand new dance.
+```
+
+Note: for plurals which do not change it's probably easier to just not use the
+plural version at all and forgo defining a custom plural altogether.
+
+## capitalization
+
+- Capitalizing the first character of a macro makes the first letter of the word
+also capitalized. 
+- Capitalizing every letter makes the whole word capitalized also, except if the
+shortcut is only one word long. 
+- For this reason, you may not want to use one letter definitions.
+- This also means that macro definitions are case insensitive and are completely
+ignored. 
+
+Say you have the following macros:
+
+```
+.hc hydrocarbon
+.h hydrogen
+.c carbon
+```
+
+Input:
+```
+.hc
+.Hc
+.HC
+.h
+.H
+.c
+.C
+```
+
+Output:
+```
+hydrocarbon
+Hydrocarbon
+HYDROCARBON
+hydrogen
+Hydrogen
+carbon
+Carbon
+```
+