README: Describe changes from upstream

Author: nosoop

README.md

@@ -1,4 +1,49 @@
 # tf2attributes
+
 TF2Attributes SourceMod plugin
 
 https://forums.alliedmods.net/showthread.php?t=210221
+
+## Differences from FlaminSarge's upstream
+
+This fork provides the following new functionality:
+
+Add / remove temporary attributes on the player (using the game's own time-based expiry
+mechanism for it).
+
+```sourcepawn
+// replicates the temporary health bonus granted by the Dalokohs Bar:
+TF2Attrib_AddCustomPlayerAttribute(client, "hidden maxhealth non buffed", 50.0, 30.0);
+```
+
+Adds the game's "attribute hook" mechanism that collates values using an attribute class:
+
+```sourcepawn
+// computes the final damage multiplier based on the given item and owner's attributes:
+float damageBonus = TF2Attrib_HookValueFloat(1.0, "mult_dmg", weapon);
+```
+
+Support for setting / getting attribute values via strings:
+
+```sourcepawn
+// set an entity's custom projectile model:
+TF2Attrib_SetFromStringValue(entity, "custom projectile model", "models/weapons/c_models/c_grenadelauncher/c_grenadelauncher.mdl");
+
+// get the name from an item:
+TF2Attrib_HookValueString("NO NAME", "custom_name_attr", entity, buffer, sizeof(buffer));
+```
+
+Setting custom names / descriptions is not possible.  String values that are set by this plugin
+are not replicated to the client — this is fine for attributes that are only accessed on
+the server, but if you set any that the client will read, the client will crash on access.
+
+## Installing or migrating
+
+This fork is forward compatible with FlaminSarge/tf2attributes — all plugins compiled for
+their version should continue to work with this one.  The installation instructions remain the
+same.
+
+1. Copy `tf2attributes.smx` to `addons/sourcemod/plugins/`.
+2. Copy `tf2.attributes.txt` to `addons/sourcemod/gamedata/`.
+3. If you're a developer, copy `tf2attributes.inc` to `addons/sourcemod/scripting/include/`
+(or the appropriate path for your compiler toolchain / project).